This is also a double edge sword in that if it's just a user-agent check the site tends to lack a well coded fallback for any functionality that is actually not

This is also a double edge sword in that if it's just a user-agent check the site tends to lack a well coded fallback for any functionality that is actually not supported and the page can silently break during usage. Not necessarily a problem for those that already know how to switch, but even just installing an extension for other people doesn't mean the sites are suddenly actually compatible. Sometimes even big sites like Microsoft Teams have this kind of problem for years at a time.