this works well with windows machines, however beyond that it could be really really flaky. especially freebsd/mac/linux are mostly inaccurate. PTR's are set on client machines aswell or in some newer IPv6 based networks you mostly get a PTR aswell. fingerprinting the tcp implementation can't work on network fragmentation and when you drop some packages through a firewall, yes this is mostly not in home networks and most home users won't use linux at all however since netflix and other streaming providers opened for linux aswell, they will actually just allow linux users to use a vpn since they can't detect it safely without false postives like they could on windows.

And MTU differs extremly between US and Europe (Thanks to PPPoE and PPPoA)

PTR is wrong. My Server is a home user and I'm a server?! Also this guy has a better database since he can detect linux 3.11 however on my home network I'm behind a proxy, thats something he didn't detected.

Edit: Oh and on IPv6 only networks with DNS64 and NAT64 you will get really aweful results if you operate on a ipv4 based service (i'm looking at you netflix)