Mixing in mono makes it (slightly) easier to balance tracks and will also reveal mono incompatibility issues (tracks that seem to get lost or disappear in the mix but are audible in stereo) or Even tracks that suddenly become a lot louder! This has been a handy trick for me in the past to help me get to my end goals, and one that I would also highly recommend. You don’t have to listen out for anything when in mono, just carry on mixing! Any changes in your tracks should be pretty obvious.
I mix in mono but frequently hop back and forth to stereo as I go. I’ve seen Warren discuss this topic before and If I recall correctly, he mixes in stereo but he also knows what to listen for and also what his track should sound like coming from his speakers because he knows his monitors/room very well.
If you are not at that level yet then I don't think mixing in mono is a bad idea at all, but in practice (in my humble opinion) it should be compulsory to listen in mono, stereo, headphones, monitors and even earbuds and car, when you start to become more experienced You won’t need to cross reference on everything so much but just takes time.
Hope this not a double post, I replied but lost it again so I’ll post again but also I’ll add that mixing in mono is supposed to force you to EQ all of your tracks better instead of relying on stereo information to create clarity between your tracks, so you would have to EQ frequencies away from an offending track that would be masking frequencies on a track you want to hear more clearly, instead of say.... panning to achieve the same goal, which could likely just lead you into a mono incompatibility issue.
This is my take on it, I won’t be offended if someone else on here disagrees with me as there are people here far more intelligent/expereienced than I.
Cheers, good luck!
Tl:dr ? A plus 1 from me for mixing in mono!