We are currently playing a 5e campaign. The races in MotM are the only things OUTLAWED in the campaign itself right now.
There is balance, and then there is inherently unbalanced from what I've seen. If this is how they are going to craft the races in the future, I think there are some default races which will stop being played. Then again, they may not include all these races in the future as the default. Some are clearly superior to others.
I am using the Monsters in the book though.
One other thing that this campaign is teaching me, I REALLY dislike having to refer to multiple books at a time for classes and other options. Sticking only to One book next time (the PHB) is looking more and more appealing. I have my 5e books that we are using in a filecase!
Available are PHB, Tasha's, Xanathar's, and SCAG's are the primary class options and that seems overwhelming at times. Using Mordekainen's, Fizban's, and MM for monsters isn't as bad (note, MotM do not include the Monsters from Fizban's) but still annoying. Perhaps the Anniversary edition will combine everything into one book again (would be a THICK book) so I don't have all this page turning at times....especially at level ups and right after to see if I understand what their new abilities and such are.