There's some valid criticism on ActiveRecord. As always, Uncle Bob sums it up perfectly:
The problem I have with Active Record is that it creates confusion about these two very different styles of programming. A database table is a data structure. It has exposed data and no behavior. But an Active Record appears to be an object. It has “hidden” data, and exposed behavior. I put the word “hidden” in quotes because the data is, in fact, not hidden. Almost all ActiveRecord derivatives export the database columns through accessors and mutators. Indeed, the Active Record is meant to be used like a data structure.
On the other hand, many people put business rule methods in their Active Record classes; which makes them appear to be objects. This leads to a dilemma. On which side of the line does the Active Record really fall? Is it an object? Or is it a data structure?
Wikipedia sums the criticism in a testability concern:
In OOP the concept of encapsulation is often at odds with the concept of separation of concerns. Generally speaking, patterns that favor separation of concerns are more suitable to isolated unit tests while patterns that favor encapsulation have easier to use APIs. Active Record heavily favors encapsulation to the point where testing without a database is quite difficult.
Specifically for the Ruby on Rails implementation, Gavin King writes (emphasis mine):
At this point, most developers are thinking um, ok, so how the hell am I supposed to know what attributes a Company has by looking at my code? And how can my IDE auto-complete them? Of course, the Rails folks have a quick answer to this question Oh, just fire up your database client and look in the database!. Then, assuming that you know ActiveRecord's automagic capitalization and pluralization rules /perfectly/, you will be able to guess the names of the attributes of your own Company class, and type them in manually.
Also on the Ruby on Rails implementation, John Januszczak writes (emphasis mine):
PROBLEM #1: STATIC METHODS
Some would say using Static methods simply amounts to procedural programming, and therefore is poor Object Oriented design. Others would say static methods are death to testability.
PROBLEM #2: GLOBAL CONFIGURATION SETTINGS
Therefore there is no dependency injection on the Account class in my example, and by extension, on the Account instances. As we should all know by now, looking for things is very, very bad!
A few more resources on why ActiveRecord and ORM is generally considered to be an anti-pattern:
ActiveRecord always felt like an extremely useful anti-pattern, but I do agree that it goes against SRP and additionaly that it goes against the dependency inversion principle.