Two of the features of Invites are sharing photos and sharing music. These are both locked down to users of Apple services (Photos and Music). So you can invite anyone, but those people won't be able to fully participate in your event.
There's nothing really wrong with Invites if you're happy to only have photos from people with iPhones or to let the music be exclusively chosen by Apple users, but you can't pretend it's a fair and equal system.
Depends on how you define locked down. Apple Music has been available on the Google Play store for years [1] and also supports listening in a web browser on any operating system [2]. I do agree Photos could use some cross-platform improvements.
There's nothing really wrong with Invites if you're happy to only have photos from people with iPhones or to let the music be exclusively chosen by Apple users, but you can't pretend it's a fair and equal system.