I suspect a major problem with ORMs is that a number of people are using them without a full knowledge of SQL beforehand. Relational databases are complex and you need to understand what's going on if you want to make best use of them.
I've never had any issues with ActiveRecord (although I agree that it's mixing of data access and business logic can be a problem) - but I also know what needs joins, which columns to include, which indexes to use, when to drop to raw SQL; all from years of writing complex SQL and stored procedures by hand (and I never want to go back to that). And I don't ActiveRecord to magically guess that stuff for me.
I've never had any issues with ActiveRecord (although I agree that it's mixing of data access and business logic can be a problem) - but I also know what needs joins, which columns to include, which indexes to use, when to drop to raw SQL; all from years of writing complex SQL and stored procedures by hand (and I never want to go back to that). And I don't ActiveRecord to magically guess that stuff for me.