They're not terrible. They were originally supposed to be an alternative telling/reboot of the Universal Century but it wound up feeling very different so people don't like it. Kind of like how the Star Wars sequels were supposed to "rhyme" with the originals.

But as they went on SEED got more and more different because the rhyme idea just want working so well so it got regarded as kind of a failure.

On its own it's a fun watch but within the overall context of Gundam it's kind of a forgotten stepchild. It doesn't suck, but it's simultaneously too much like UC Gundam and too different, like if you tried to copy a friend's homework, but that friend had copied yours first.