It's a lot of unnecessary process and meetings. Daily standups, biweekly planning meetings, retrospectives, endless backlog grooming sessions etc. I've been at companies that burn 10-20% of the team's time each "sprint" in unnecessary meetings to discuss work rather than actually doing work.
The meetings impose heavy burdens on anyone who doesn't want to be interrupted at that time. A typical 10am standup meeting is too early for night owls and too late for parents who drop their kids off to school at 7am, as there's a useless 3 hours between school and standup where you can't get anything done because there's a meeting soon. Scrum amps up context switching which prevents people from getting into the zone. Having lots of meetings and interruptions shreds your day to bits.
Then there are the Scrum-specific tools. Rather than using modern project management tools that get out of your way like GitHub or GitLab, you've got regressive, redundant nonsense like Rally or Pivotal Tracker being thrown around. All so you can play planning poker in the cloud. Shoot me now.
Defenders of Scrum tend to use no true Scotsman arguments. "Oh that wasn't Scrum." Or "that was Scrum being done badly." And so forth. Meanwhile the number of companies and teams doing it "correctly" seems to be few and far between. Perhaps because "correctly" is such a fuzzy thing to define. Means different things to different people. Seems to me where it "works," teams are successful in spite of Scrum, not because of it.
Basically it's popularly understood that Scrum basically exists to micromanage the shit out of dev teams. The managers who like it are the shoulder tap types whose management style is straight out of 1950. Marshall McLuhan is rolling in his grave.