There are lots of people pretending to be Google and friends. They far outnumber the real Googlebot, etc. and most people don't check the reverse DNS/IP list - it's tedious to do this for even well-behaved crawlers that publish how to ID themselves. So much for User Agent.
> So much for User Agent.
User agent has been abused for so long, I forget a time when it wasn't.
Anyone else remember having to fake being a Windows machine so that YouTube/Netflix would serve you content better than standard def, or banking portals that blocked you if your agent didn't say you were Internet Explorer?
I mean forget that, all modern desktop browsers (at least) start with the string 'Mozilla/5.0', still, in a world where Chrome is so dominant.