Hmm.
Everything I’ll say is my own opinion and it does not reflect the opinions or stance of the Pencil2d Project team members, so basically I’m speaking for myself here FWIW.
No matter how I answer this I’m sure you are quite possibly going to take it in the wrong way, so apologies in advance, I don’t really have beef with you, but today I simply don’t care enough about being empathetic.
I always feel there’s a fundamental misunderstanding on how open source projects work, as opposed to big fat bucks programs, so here, have a wall of text. There’s no TL;DR. It’s up to you if you want to read the rest, but if anything at least read the “closing thoughts” section. Peace 
The Pencil2D Project
Let’s get some small facts on display: Pencil2D is a project that’s not sponsored by anyone but the people that work on it. Everyone that has had a hand in shaping the app has been an unpaid volunteer working on their spare time. Keep that in mind.
Contributors tend to come with all manner of intentions and ideas for the project, but overall, and this is true for any project that’s self-funded, people work on whatever they want whenever they can.
We do try to discuss guidelines and general needs for a new version of the software, but you’d be surprised to see how hard it is to ask someone to implement anything without life coming up and messing any of us up. For example sometimes I end up having to work triple shifts on my day job; you think I can even think about Pencil2D? well I try, but it’s not easy.
How Releases Work
The way releases were structured some years ago is that there’s usually a version dedicated to fix things (bugs), and another one to add things (features). The next version to come is incidentally the feature one.
From version 0.6.6 it’s been almost 1 year and half up to the most recent development build, which has seen a shit ton of bug fixes, but don’t let that fool you, in software development no matter what you do, it’s impossible to get a perfect build. Many times you fix one thing, and other three break. It sucks, but the devs try to fix these “regressions” the best they can.
Right now some things were improved, but others broke quite literally.
Broken features (?)
Copy paste frames
Also you ask for the copy paste? bro, we all wanted to have a better way to copy and paste and only until some french dude managed to make some breakthrough was it possible to advance that, he then left the work unfinished and one of the devs had to pick it up until it was implemented a few months ago.
You can now copy, paste, and delete multiple frames easily from one layer to another. It’s only the first step towards making it more flexible so at some point we can copy multiple frames, from multiple layers.
Anyway, If you’ve tried that and you think it’s buggy you need to be more specific and describe how it break so the devs can fix it.
Camera handling++
you realise most shots in animation are a locked off still frame anyway? With no camera movements?
You say that because you don’t have the full picture (pun intended). The camera project is the result of pro animators and novices asking for a better way to handle the camera. This will also benefit the drawing transformation widget among many other things as well as getting improved motion interpolation control for the camera and a motion path guide.
This has been sitting for years picking dust, however only until last year someone managed to work on it because there were a ton of problems surrounding the source code that had to be fixed first. Imagine that.
Tweening, and more tweening
Now you ask about tweening. Sure tweening is helpful, but why do you think no one has been able to implement it so far? because it’s hard with a capital H(ell).
You may think that since the camera has tweening, it’s easy to just add it to other layers, well, unfortunately that’s not true.
As you may know there are two types of tweening. Motion tweening and shape tweening.
For shape tweening Pencil2D requires the vector engine to be remade from scratch to accommodate for a better way to handle and manipulate vectors.
For motion tweening vector is not a requirement, but to work for bitmaps (tweening the layer) the bitmap engine needs to be modified and the timeline also will need to be rewritten (AFAIK) to handle how the tweens should be represented to the user among other things only visible for the developers (like the actual source code classes that can mingle with the other layers).
Incidentally the bitmap engine is currently being changed to something that supports brushes and a lot of properties, and it will land on the software probably next year if nothing else breaks. This is because people have been begging for better drawing tools for years as well.
A word on moving frames
But we cannot simply move frames, copy and paste frame…etc. without it glitching out.
In the dev versions 
You click once. You select. Click a second time & drag the frame. The frame moves.
FYI this is the same behavior as in Flash. You click to select and then you have to click again and drag.
If it doesn’t work for you, you need to specify WHY and HOW it doesn’t so the devs can trace back their steps and find the culprit to the issue.
Salted opinions 
waste of time
Seriously? Come on dude, we’re not fucking Adobe or a mega corp with truckloads of money and 400 employees waiting to be laid off.
Just try going and complain on their forums and see if they care for your issues at all.
Actual fucking pros have ended up begging for help on tight deadlines and they are met with scorn and no help whatsoever despite paying for their products and being loyal customer for years on end.
I’ve used Macromedia Flash / Adobe Animate for 22 years for example, I’m even using it for my day job right now and I HATE it, it CRASHES ALL THE FUCKING TIME. They simply don’t fucking care.
We at least try answer all the questions and requests ppl bring to this forum, but sometimes it’s just pretty much impossible to implement anything beyond patching band aids because the spaghetti code that was left behind by the predecessors.
To my understanding originally the source code was so botched that one couldn’t help but curse in disbelief since the amount of time it would take to do complex stuff is disproportionately nuts compared to how it would be if it was made from scratch in a better way.
The devs have been trying to improve it for the past 9 years, but they can’t help but tread lightly unless you simply want to mess up the code so hard that we end up with a worse problem than before (and this has happened before)
It’s also important to consider that the devs have their own families and day jobs and while I can’t speak for them in full, to me it’s just anger inducing to get slam dunked every time they try to make something new because they managed to find time and wanted to implement it, but oh lord it wasn’t the fucking text tool every 10 year old kid wanted just to make shitpost memes on YT.
You may think something is simple because using it looks simple to use, but in fact making it is the complete opposite.
Closing thoughts
Not saying this all applies to you, but to be honest the worst part of anyone coming to just let out their frustration bc Pencil2D isn’t Flash or Harmony, is just sucking up insults, bad vibes and that better-than-thou attitude of internet trolls, year after year, day after day.
Right now there might be better free apps for you to use too, though they come with their own problems, it depends on you how much shit you can withstand so you can produce more. Want tweening and professional grade software with a ton of bugs (that are getting fixed relatively fast?) then go and use OpenToonz.
If the Microsoft Flight Simulator interface worth of controls doesn’t turn you off and you don’t experience crashes because your computer is properly configured, then I actually recommend it. There’s people doing Freelance work for japanese studios with that and it works great with Clip Studio Paint.
Too bad we don’t have a Japanese conglomerate behind us like they do, but heck, can’t always choose a winner side.
p.s Excuse the language if you’re offended by that kind of thing.