I play both cello and E-bass live with rock bands.
For cello (this would probably extend quite well to most other acoustic instruments) in-ears are nothing short of a blessing. With conventional monitoring, feedback is always a very serious threat. With in-ear it's seldom an issue at all. I can always hear my own notes excellently, which makes intonation a lot less dangerous, and prevents a “need to play everything loud as 𝆑𝆑***” feeling.
The only problem I sometimes do have is to hear other instruments: in festival settings or when playing with larger ensembles, engineers tend to be unwilling to give me a monitor send all for my in-ear, so I often do my own “mix”, which has basically only cello in it, plus a master mix if I can get it. Often, I would then like a bit more keys, guitars or hi-hat for precision and a context to intonate to. One possibility is to place up some dedicated microphones on stage for this purpose only, but this is a hack and really not practical. What I do now is just keep my own levels quiet enough so I can still hear everything from the outside. It feels a bit weird to be on stage but not hear any loud sound, but my ears certainly are grateful for this, health-wise†.
However, when playing bass (again I suppose this somewhat holds for drums and electric guitar too) then monitoring is a lot more about feeling. No matter how you set up an in-ear mix, it won't give the groovy experience of a nice 6x10" cabinet behind you and a good helping of bass-drum from a wedge or PA subwoofer in front. So even when I have my in-ears on while playing bass (usually not), I don't actually use them at all for monitoring, just as passive earplugs.
Interestingly, I never perceive such a lack of “feeling” when playing cello with in-ear, even if I perform a contrabass-like part on it. I suppose this is because the cello is such an “embrace” instrument, I feel lots of vibration from the body (which also picks up sound from the air... once I played with a metal band, sitting next to the drums the double-bassdrum made by bow jump on the strings!).
As for singing: I don't do that a lot. My in-ears are closed, so I hear my own voice very loud in my head. When I do supply a few chorus vocals, this is actually quite useful (I just need to sing somewhere in the vicinity of some mic, and hear myself without any actual monitor routing). But it's not a good monitor sound for getting voice nuances right, and when you're singing all the time this would just become really annoying, so if you're a singer you should get an “open” system. Never tried one of those, but I wager it's worth to check it out! I've never seen an engineer to reject a lead singer their own monitor mix.
(Indeed I believe as in-ear becomes more commonplace and digital consoles with loads of buses become standard, engineers will in the future routinely give all musicians their individual in-ear channel: not needing loud stage volumes is a blessing for the mix as well.) It is nowadays also possible even for a small band to make themselves completely indepent of any monitor mixer, by using a split box to a separate rack-mount digital console, with which everybody can set up their own monitor sound (using a tablet or phone for dialling in the level). This is extremely useful especially when there's little time for doing a proper sound check.
†I should say that I used to frown at the very idea of using earplugs in any music setting – “if you want to hear less, then what's the point of hearing music at all?” I still think it's absurd if the audience or engineers need earplugs, but on stage this is sometimes just rational.
(On the flip-side of ear-protection: when they are too loud, in-ears can actually cause serious damage! Be sure to use a headphone amp with limiter.)
