It's not necessarily that the ORM will give you bad results, it's that an ORM might be giving you sub-optimal results.
A lot of ORMs don't use more advanced querying functionality, and SQL can be a very harsh taskmaster when it comes to decisions about how to structure a query. So on an enterprise-grade DBMS it's not unheard-of for hand-coded data access layer to outperform what an ORM does by multiple orders of magnitude.
Of course a good ORM offers you a way to substitute your own queries. Unfortunately teams that rely heavily on ORM aren't necessarily cultivating strong database talent, so these kinds of heinous performance drags can easily end up going undiscovered and unfixed even though the tools to deal with them are readily available.
