I can sort of see the argument behind Liara being a "writer's pet".
Now, I don't have anything against Liara specifically. She's not among my favorite characters in the series, but I still like her just fine.
But the way she was used/written certainly can come across that way. She was stretched into situations that didn't necessarily fit which is also why some people complain that her personality changed; it didn't necessarily change, it was just that she got kinda pidgeonholed sometimes and I can certainly see how it came across as favoritism. Whether it was favoritism and not just sloppy writing or bad characterization, I can't say.
I do not see how it applies to Lelianna, however. I think a great deal of it stems from their retconning of Lelianna potentially dying in Origins. While I can see why the retcon can distress people (one of my own major problems with DA2 was the amount of retconning they did), I think labeling her a "writer's pet" is a misplaced notion. Just because the character has been used more often than other characters doesn't inherently make it favoritism. I think Lelianna has been utilized pretty well in the DA verse. Though I admit that this opinion is perhaps slightly ignorant, as I have not read all of the books.