I've used them as a proxy for a web app at the last place I worked. Was just hoping they'd block unwanted/malicious traffic (not sure if it was needed, and it wasn't my choice). I, personally, didn't have any problems with their service.

Now, if you take a step back, and look at the big picture, they are so big and ubiquitous that they are a threat to the WWW itself. They are probably one of the most valuable targets for malicious actors and nation states. Even if Cloudflare is able to defend against infiltration and attacks in perpetuity, they have much of the net locked-in, and will enshittify to keep profits increasing in a market they've almost completely saturated.

Also, CAPTCHAs are annoying.