Apple ID with iCloud email vs non-Apple email, any differences in capabilities?
are there any other differences between an @icloud.com Apple ID and a 'non-Apple mail' Apple ID?


No. Both approaches give you a fully capable Apple ID. There's no different type of Apple IDs.


  Any gotchas down the road, like some security or health related synch services not working with non-iCloud accounts, or less cloud space, or...?


No, nada. Any Apple ID created using whatsoever approach is similar in capabilities and security.

Note: A difference that I noticed: I attempted to create an Apple ID using a 3rd-party email and created it via Web. You only get 1 GB of iCloud storage.