It doesnt need to be 100% reliable, just reliable enough.

If certain websites fail to be detected, thats a security issue on those specific websites, as I'll learn which ones tend to fail.

If they rarely fail to detect in general, its infrequent enough to be diligent in those specific cases. In my experience with password managers, they rarely fail to detect fields. If anything, they over detect fields.

I think this security model requires nontechnical users to be paying more consistent attention than is realistically safe to rely on.