I think it was in an Imaqtpoe video that like Tryndamere or one of the devs said something like "I really like what we did with the stopwathc mastery, where we basically said "Here we'll put all of the power of a major rune into one active item use, and if you use it well...welp, there went that mastery".
Stopwatch has a lot of potential, but that potential is super telegraphed and super dependant on the user. I can't count how many times I've ganked or all-ined someone, they used their stopwatch, and then died anyway, and...that was their rune. There it went. All gone!