Perhaps the problem the OP is referring to is code like

    foreach($things as $thing) {
      $thing->setValue('new value');
    } 

and then looping through 10000 rows to do that, instead of using an UPDATE query.

I think the best criticism of ActiveRecord (and ORM in general) is performance, but it's only relevant on medium-to-large websites. It is a fantastic design pattern for prototyping, and good enough for production on small-to-medium websites as long as there are caching layers above it.