Honestly, I think people tend to overcomplicate the Copy quirk's limits. Monoma's ability is pretty straightforward when you break it down. He can only hold one quirk at a time, and even that is temporary, lasting somewhere around five minutes before it fades. That single-user restriction is a massive tactical limitation in a team fight; he has to touch the person whose power he wants, which isn't always possible, and then he's stuck with only that one toolset until the timer runs out.
What's more interesting, and often overlooked, is the psychological and mastery limit. He doesn't just get the quirk at its user's level of skill. He gets the raw power, sure, but he has to figure out how to use it on the fly. If he copies something complex like Creation or Explosion without understanding the mechanics, he could hurt himself or be wildly inefficient. The pressure to master a completely unfamiliar ability in the heat of battle, under a ticking clock, is arguably his biggest hurdle.
It's not about raw power storage; it's about his adaptability and quick thinking being the real engine of the quirk. That's why he's such a good strategist—he has to be.