I'm going to talk about Active Record as a design pattern, I haven't seen ROR.
Some developers hate Active Record, because they read smart books about writing clean and neat code, and these books states that active record violates single resposobility principle, violates DDD rule that domain object should be persistant ignorant, and many other rules from these kind of books.
The second thing domain objects in Active Record tend to be 1-to-1 with database, that may be considered a limitation in some kind of systems (n-tier mostly).
Thats just abstract things, i haven't seen ruby on rails actual implementation of this pattern.
