Good UI can only positively effect backend and architecture, largely because it forces a good cognitive model and thus relationships between entities. A "Good" UX can have a disastrous effect on backend and architecture. It may put undue focus on glitz and cost the performance or reliability of the application greatly. Those LEAD to a bad UX, but that's typically not in the UX focus up front. Most probably think it's an obvious default, but unless focus is put there, it doesn't come for free. Zero-sum game, paying the glitz tax will cause a debt somewhere else.