Nah. It doesn't suck. It's just terribly overrated. The animation is great, but it's ham stringed by ugly over-stylized character and mech designs. Character development is inconsistent in characters other than Simon and Nia. The writing is hit and miss, the writer while technically proficient, lacks heart. He gets the main conventions of a super robot show right, but never captures it's spirit.
And despite the highs of the series, GL ultimately fails due to episode 27. The most overrated episode of the series. It lacks the visual punch of ep 15, the Nia and Simon angle is overplayed just to create sympathy when they write Nia off, and there is a drastic change in tone that conflicts horrible with the rest of the series.
So good show, but too many flaws. and that is how you troll. Calling a show overrated gets a better reaction than just "shit sucks"