Combine if they are independent pieces of software, yes. Whether linking (hard or soft/plugin) counts is a matter of legal uncertainty. The FSF has consistently said that a single function call into a GPL library constitutes a derivative work, but other legal experts disagree [1].
Certainly, if I maintain a repo which calls a function in a GPL library, it would be misleading for me to say that my repo is MIT-licensed. I could potentially say that my specific code bits are MIT-licensed but that (per the FSF's interpretation) if you import my library, your resulting artifacts are GPL.
That is quite hard to communicate, and it also rests on a particular interpretation of the license that is the most common one but isn't universally accepted. It also becomes tricky with distributed copyright ownership without a CLA, the way most open source projects are run. I think that this rises to the level of "impossible to use".
But I guess one specific point here is that if you download my MIT-licensed-but-links-to-GPL repo, and rip out all of the bits that link to GPL libraries, you can use my code under MIT.
Compare this to MPL 2 which has none of these issues, and very little legal uncertainty in general. There is still some uncertainty regarding what constitutes a copyrightable work, though.
[1] https://tech.popdata.org/the-gpl-license-and-linking-still-u...