I've used a couple of different ORM products, but no NHibernate. My major concern is similar to yours: there may be nothing preventing you from using them robust database designs, but that doesn't seem to be the normal--or even intended--use. Anybody who's familiar with my work (or even my comments on forums!) knows that I'm a long way from being a database wizard, but at least my work is based on the assumption that the database needs normalization, relationships, constraints, and indexing.
On a related topic, I see too many commercial products, even from major publishers, that fail to implement what I (and apparently you) consider to be the bare minimum sound design principles.