I think there are few reasons why most would seem to be negative towards the group of plecs collectively known as 'commons'.
They are usually inappropriately sold, usually told they will devour algae, even eat fish waste and not grow big. The are usually quite dull in colour (although I've seen some that were magnificent). And that they are common.
In their defence, they are usually the reason why everyone ends up in pleco hobby (I know I am, I have a 15 year old Pterygoplichthys gibbiceps). They are very cheap and easy to look after, providing you can look after them properly.