This fantasy, I find, is dangerous.
Done with an unknown master, it could lead to a bad snuff murder.
("Hey, she said no limit... " <evil evil evil grin>)
Done with a known master, if known-well, well, you might know enough to guess the limit. So that's only half-surprise.

Unless both words out the master's limit first, and just like a top work on/within the limit of the bottom, you might have some special session where you remove all your limits from impairing the top one. So the top is free to go any way.

But anyway that's sound a bit like playing without a safeword, and if the top is not to care, I find it especially disturbing. External supervision might also waste it.

Just my 0,001 cent. (I'm lacking more money)