Well there are definitely servers that don't federate with Mostr or Momostr... But there are other servers that do, and posts don't show up... So I think there are other issues.

As for spam, yeah there's some, but there's good content too... I've been enjoying nostr:nprofile1qqsyk3sjx32e4xs5qj2rtcwapeyrj95yfn4d4h7kkhdgyjc409fmmespr9mhxue69uhhyetvv9ujumt0d4hhxarj9ecxjmnt9urzqpjr for instance