As a musician I'd say Whiplash wasn't that bad in terms of believability, musicians have pushed themselves that far in the past for their goals. Flea of red hot chili peppers would be bleeding profusely from his hands during a show and he'd put glue on the cuts, Eddie Van Halen would lock himself in his room for 6 hours to practice no exceptions. Dave Mustaine recovered from near full paralysis of his playing arm and multiple neck/throat surgeries, James Hetfield was nearly burned to death. Music being such a competitive and emotional field makes it inherently prone to strong personalities and displays of extreme fortitude.
Dave Grohl's guitar
Zakk Wylde mid performance
Beethoven's dad Johann treated him like a slave and would beat him if he didn't practice enough, Whiplash imo isn't that far fetched. A tyrannical bandleader looking for perfection? Seems like that's the norm, not the exception. I'm sure Hetfield would have chosen bloody handed drum playing over burnt to a crisp.