I accept that someday I'll give up MH and move to IMAP mail clients
My current email tooling is strongly built around MH, using both command line tools and exmh. MH assumes a traditional Unix mail environment where your inbox can be accessed through the filesystem, and more than that it fundamentally assumes that it entirely owns your email. As many people who try it out find out to their regret, MH's only interaction with the regular Unix mail ecosystem is to get your mail out of your Unix inbox as fast as possible.
So far I've been able to use and keep on using MH because I've
worked (and had my personal email)
on Unix systems that handled email in the traditional Unix way,
with your inbox directly accessible through the filesystem in
/var/mail and so on. However, these are a vanishing breed, for
reasonably good reasons, and in the modern world the generic way
you get at your email is IMAP. IMAP is not very Unixy, but it's
what we've got and it's better than being stuck with proprietary
network mail protocols.
MH and IMAP not so much don't get along as don't interact with each other. As far as I know, MH has no protocol support for IMAP, which is not surprising; IMAP is designed to keep all of your email on the IMAP server, which is completely opposite to how MH operates. It might be nice to have an 'IMH' system that was a collection of command line programs to manipulate IMAP mail and folders, but no such thing exists that I know of and it's unlikely that anyone will ever write one.
Some day I will have to use a mail system that can only be accessed over IMAP. In theory I could deal with this by using a program to pull all of my email out of IMAP and hand it over to MH as local mail; there are a number of things that will do variants of this job. In practice my feeling is that doing this is swimming upstream against a pretty powerful current, and thus generally a mistake. Certainly I expect that I won't be energetic and annoyed enough to do it. By that point MH will have had an extremely good multi-decade run for me, and very few programs last forever. I can change.
(Also, by that point I expect that I will be really tired of how MH and exmh don't really deal all that well with HTML email, because I expect that HTML email is only going to get more and more common from now onwards.)
PS: The clever reader will have guessed that I don't currently have a smartphone or other limited secondary device that I want to read my email from, because all of those really want you to use IMAP (or something even less MH-cooperative, like GMail). That may change someday, at which point I may have to re-think all of this.
Sidebar: I don't see this happening any time soon
Locally we're still very strongly attached to filesystem accessible
procmail and all of the other artifacts of a traditional
Unix mail system. There would be quite a lot of upset users if we
took that away from them, so I don't expect it to happen short of
something truly drastic happening with our overall mail system.
(Nor is the department likely to give up running its own email system any time soon.)
As for my personal email, well, that's tangled but certainly my attachment to MH complicates my life. There are lots of places that I could get IMAP mail, probably even IMAP mail for a custom domain, so if I was happy with IMAP alone life would be quite a bit easier. Until I de-tangle my personal email it gets a free ride on work email's MH friendliness; after I de-tangle it, I will probably still run my own servers for it and so I could run MH there if I wanted to.
(At that point I might want to switch to IMAP for various reasons.)