Domain management in Blesta works if your customers have one domain they've bought at the same time as their hosting or if they've transferred the few they own and don't care about maintaining them (leaving that ICANN responsibility to you). It fails when you have a dozen+, spread across different registrars and if you're selling more exotic domain names (with the registration restrictions).
The biggest problem though is the lack of self-service option on the client side. It should be as good as what Enom or Logicboxes has to offer. Customers should be in control of their domains (contacts, DNSSEC, EPP, privacy protection, trustee service, etc.). Blesta should just sit one level above registrars, offering the huge benefit for customers of having only one system to use to manage all their domains.