As someone who enjoys playing monks a great deal, I think that most of the criticism is misplaced.
They are amazing for what they do, but they aren't a front-line, exchange blows-type class like a Barbarian or a Fighter.
They are skirmishers. They use mobility to their advantage.
Until level 5, Monks have a +10 to movement. That's what the spell Longstrider gives. It's okay, but not exactly game breaking. Monks get +15 ft. for level 5-9, which is a tad better but hardly game changing.
Monks pretty much have to be in melee to attack. To get in and out of combat and make use of their mobility, they need to burn 1 Ki point and spend a bonus action each time they want to do it. This brings down their DPR considerably.
Honestly if you want a high priority target taken down, you are way better off attacking at range. A Samurai, Battlemaster, or Gloom Stalker archer with Sharpshooter is going to do more damage more reliably than a Monk can ever hope for.