I've actually found that a lot of devices just ignore an invalid (ie not from a trusted CA) certificate for this. Android in particular will happily continue with no prompt to the user that the cert is not trusted. I even had it somehow forget the CA that I specified with the network credentials. I'm not 100% certain on this, but I vaguely remember having an issue with Network Manager also not validating the server certificate with TTLS.
It's just too risky where a device could decide either for "convenience" or incompetence not to notify about an invalid server certificate and go on to divulge that device's login credentials to the MITM. Or a user not configuring a device properly. I don't have to worry about that with regular TLS, it's enforced on the server and if it's invalid it won't connect, period.