Imaro
Legend
Or, as another example, picture this scenario. You're Bob the melee fighter and you've put hundreds if not thousands of hours into your character. You are finally reaching the higher levels. Jim the archer however in your group has decided to pick up Sharpshooter and Crossbow Expert. As you start leveling up, you start to note a lot of encounters you're pretty ineffective compared to Jim. Jim can basically do everything you can do up close and personal in combat, but he can also do the same things at range, and unfortunately for you, most of the higher level encounters you're facing happen to use a lot of highly mobile flying, or legendary moving creatures.
Poor old you is huffing and puffing back and forth, throwing a Javelin here or a hand axe there, trying to keep up. Meanwhile Jim is shooting crossbow bolts like lazers, completely ignoring cover, and doing a ridiculous amount of damage from hundreds of feet away. Soon the party basically stops buffing you completely, and they start saving their buffs for Jim, realising how much more effective he is over you.
Unless the DM start banning feats and housing ruling things, which is probably going to be very annoying for Jim, OR engineer encounters to give Bob something to do (and subsequently nerf Jim, also pretty annoying for him), you're going to end up with a serious imbalance at your table.
I'm confused by a few points in this example... maybe you could clear them up for me...
1. What exactly did Bob the melee character take when Jim took Sharpshooter and Crossbow Expert?
2. Why is the DM using a majority of flying creatures (without having told the PC's this beforehand so they could build appropriate characters) as opposed to a mix of encounter types that allow everyone to shine and have fun... I mean this has been a "problem" in every edition of D&D... certain monsters cater to certain character types more than others... (as an example undead monsters favor paladins and clerics doing more damage)? These monsters are there to let different character types shine and to make the game slightly more difficult for other types but using one type of monster exclusively is not an imbalance in the game, it's an imbalance in the DM's created threats/challenges.