That huge difference between your promises and real app make people frustrating, especially those who paid for it in order to support your team. It is super slow even on new macbook pro. It has a dozens of bugs. It is not stable, app crashes quite often. You can't use Mendeley with this app in a normal way, developers made their best to force you to use "Papers" which is not free again, so you can not just insert citation from Mendeley, you have to save it first in another special format, and then import it into the "recommended app by the Manuscripts team". Also export options of Manuscripts are very poor, it con not be an alternative to any know app, and the use of it is just an act of masochism. You just stealing money from people, you should be shamed of what you are creating. I'm full of regrets of buying this app and supporting you. I will recommend for every one I know never use your products.
Manuscript - the most disappointing app
There's a lot of stuff Manuscripts gets right; the figure panels are better than any of the competitors (outside of doing it all in Prism, but that can be a real faff), the interface is nice for getting actual writing done without becoming one of those incredibly pretentious minimalist things (hi, IAwriter!), the tables are surprisingly decent (compared to, say, Scrivener), the inline references are really nice and the equation support is great.
I sympathise with your frustration, because there are still a lot of annoying things - like the fact that copy and paste still don't work properly - and having a nearly-great experience spoiled is much more irritating than something that's just plain mediocre.
I've managed to write a few publishable things in Manuscripts, so I don't think it's fair to call it unfit for purpose. That said, my thesis is in Scrivener and is staying there until Manuscripts learns to stop breaking its own Preferences window.
I still like the idea of this app. My only concern is that it crashes very often. So it is almost not usable in a stable way. I don't suggest the developer put major effort to add more functions . Instead, should focus on making the app stable and reducing the crashes to a lowest level.