Well, I installed Maildroid free edition and added a couple of my email accounts. These comments are, of course, just my opinion, based on very short usage and I'm no genius. I've probably missed or forgotten some stuff listed below so some could be just wrong. But first impressions matter and these are mine. I had not used Maildroid for at least 3-4 years and forgotten so much I felt almost like I never had.
Maildroid has IMAP options well covered and while I didn't try POP3 I'm sure that's covered too. My previous comments on POP3 still stand. Just from a short trial, I found little missing. I don't use Exchange so I can't comment on that.
Overall, IMHO Maildroid needs work on usability more than adding functionality. It's capable enough. Has lots of options, some that I haven't seen in any other client. Oh, there's always gonna be requests for additional, often rather obscure features. I've listed some below.
But that's not what bothers me about Maildroid. Here's the first things that struck me as needing improvement, in no particular order of importance:
1. The UI is just not attractive. Sure, that's subjective and not important for functional reasons, but to me it simply is not as good looking as it could be.
2. Color selection is limited. Custom colors available only for account colors so far. More areas should be color customizable.
3. Individual setup for accounts. I haven't found much yet. Connection management, scheduling, caching & indexing. There must be more but they're kinda tricky to find.
4. The Navigation bar is clumsy to use. Just not intuitive. I still haven't found how to show a list of folders for one account at a time. Needs to be rethought from scratch.
5. Check boxes use way too much space.
6. No # of messages to sync. Sets days instead. Just my preference to have the number of messages instead of days.
7. No # of messages to cache. Sets days instead. As above.
8. Signature moves when writing email. Bug? When I set a signature, it moved around when writing an email.
9. Speed! Speed! Speed! Simply not as fast as other clients. I have a ridiculous fast connection. MailDroid just couldn't take full advantage of it.
10. Unstable connection. Kept losing the connection. ???
11. Integration with Lightflow, Tasker, etc. If it has this I haven't found it yet.
Thanks for this, helpful, let me go through some of them.
1. Fair and subjective. I won't argue because this is so hard to do correctly, but I hear you.
2. Look under menu, settings, theme, base color and accent colors
3. There is a ton, what specifically are you looking for though?
4. Agreed. We are on this right now, this is ne of the main pieces of feedback over the last few months that has come to us
5. That is if you show them. By default, we hide them and you can swipe them in and out and also doing this you can swipe down and up to swipe many at a time. I guess if you always show them it may take a bit more room. I'll look at that
6/7. Good feedback, thank you!
8. No, we just changed this from feedback from users. It moves around and when clicked on will pop up a dialog allowing a quick change. We have always had a signature per account or global signature option, but we had just released ability for multiple signatures per one account if you like since some people said they use one account for business and personal.
9. Well, this is a tough one. Speed in what sense? Where was it slow? To sync? To get new mail? To delete? Your connection speed is not always related to how fast a mail client works (as I know you know

It could be your mail server as well. What mail server was slow?
10. See point 9, what mail server? Also, I assume it was IMAP? This could very well be a bug on how we connect to your mail server. Feel free to email me directly if you don't want to share that info on a public forum.
11. Yep, that was pointed out in an earlier post here. We are going to integrate tasker and I will look at Lightflow as well.
I should have noted to you that after we do a crypto release tomorrow, we specifically have on our plan the next release to be a usability release. I find it interesting how that was your first point! We totally agree. We had always spent time on doing cool stuff and features, but we learned now that it's hard for some people to find or use certain things!
Thanks for this great feedback!