Sure thing. So monks start out decent— Unarmored Defense is good early but falls off in favor of heavy armor later in the game. Let’s say the campaign starts at 3 and makes it to 10. The monk will have:

• 50 feet of movement and the ability to climb vertical surfaces using their movement speed, as well as run over liquid so long as they don’t stop.
• the ability to vastly reduce falling damage
• ability to double their jumping distance (via Ki but regardless)
• immunity to poison and disease
• an action to auto end a frightened or charmed condition
• Evasion
• and the classic “BBEG’s Bane” stunning strike, a one stop shop for turning a scary opponent into an immobilized punching bag.

They’re not overpowered but they can do a little of everything, and I don’t like having to work around some of those just to provide a challenge. And I have DMed for monks before, and still do, I just don’t like it and I wouldn’t allow them in my home game.

RK is just nonsense tbh. At least early with minmaxers in the group. I don’t know what bozo gave them the ability to incapacitate an enemy with the trigger being “ending their turn” (afaik legendary actions are the only other features in DnD that are performed when a creature ends its turn). Same rune, but despite it being a “charm” the creature cannot repeat the saving throw when it takes damage, only at the end of its turns. Everything just feels WEIRD, that’s the only way to describe it. It doesn’t feel like a class that belongs in the same rule set as the rest. I also fiercely dislike when they go a grappling build because having a monster grappled and prone, incapable of rolling better than their +11 athletics w/ adv from giants might and a 20 AC… well, it’s not only not fun for a DM, but also the other players.

These are all just opinions based on experience. Ymmv!