[mark-r]

What The Font does an amazing job sometimes, but it's not infallible. I'd say it misses more often than it hits.

Web crawling seems like a great idea, but probably not worth it just to get a list of customers. And it will never tell you who's using it in their app, or for signage.

[Fnoord]

What The Font was an example. I used multiple tools to find an obscure font. It was my first time trying to search for 2 fonts whilst I only had logo where the 2 fonts were used. One was relatively easy, the other one not. It cost me some time because like you say not every tool is as good as you'd expect it to be, but I did succeed.