Can you do anything you want at someone else's startup when startups are supposed to be frugal and focus?

At someone else's startup you're often required to do a variety of different things, which provides the sort of intellectual stimulation that extremely differentiated larger companies lack.

Yes, variety of different things that most of the time didn't yield the best solution.

Pardon me for being a bit sarcastic. It is what it is.

To clarify: This is why I'd prefer to start a startup. You might not succeed, but at least you're working on the project you want to work on.

Makes absolutely perfect sense. Working at someone else's startup sucks unless you're given full freedom; which is rarely the case.