Having class abilities which monsters are immune to is unfun.
5E is not unfun.
Therefore Monks can stun everything, Rogues can sneak attack everything, etc.
I rather strongly disagree with this generalization and evaluation.
Which is not to say that I have the slightest dispute with your personal preference.
But I think using your statement as a truism puts a bad restraint on the level of fun that can be had.
"Being" the monk running around stunning things is very cool and very fun. The ghost, for example, has to opportunity to be a new challenge. If you stunned 12 kobolds yesterday and you stun 3 ghosts today, how long does that stay interesting? And I say that with full recognition that there is vastly more to the game and differences between ghosts and kobolds. But all this approach does it make the differences equal to n-1.
Running around hitting an endless series of nails with the same hammer is, imo, unfun.
Adapting and winning anyway, that is vastly more fun than "I stun the ghost".
Now, 5E is certainly vastly more symmetrical than most prior editions. For the most part every class is capable of dealing with threats one way or another. 1E through 3E/PF certainly had some elements of distinct rock/paper/scissors match-ups. Whether you love that or hate that is a matter of taste. A many times these situations just end up being the time when some other character gets to shine. And that's ok so long as a good DM keeps everyone shining. If you want this in 5E though and you go tweaking things, keep that symmetry in mind. Balance it somewhere.
But, IME, doing what your character does is "fun" and people go home saying "good game". But the time when the monk beat the ghost even though he couldn't hit him, much less stun him, those are the events that people still raise their voice when they talk about it four years later.
IMO having powers that take out monsters is very fun. Having monsters with immunity to those powers though is where the megafun lives.