Sure, they may block the most basic of basic bots. But for most languages in which bots are made, there are ready-made libraries for captcha solving that are essentially plug-and-play. To move from “basic” to “makes captcha useless” is an install and about 2 lines of code.

Given that, I highly doubt it blocks bots as effectively as it blocks actual users.

AFAIK all captcha is good for these days is training data.