I agree it is important to keep this in mind. Often when you consider class balance, its important to keep in mind the "DM hooks", the ways that Dms can set an encounter to make a class shine.Deflect missile is an always-on defensive option. People ignore it or see it as not very relevant since they think that nobody will bother shooting the monk, but that makes it even more powerful than it already is.
A heavy ranged attack is a common encounter in the DMs playbook, and its a chance to let the monk strut a bit, as they do have by far the best defensive ability against ranged attacks.