They're good, there's been a couple instances where the plastic tolerances made assembly just a touch tricky, but not impossible. On Rayearth, it was one of the four shoulder gems, it just would not stay solidly in place, so i eventually just snipped the little tabs off of it and superglued it. On Windam, I'm not sure if the head assembly had odd tolerances or if I just didn't clear enough sprue flash, I just know it was frustrating because the head is tiny and very pointy.
Gradion and Rayearth both have limbs that like to slide apart with basic posing if you're not paying attention (Gradion's thigh swivels, Rayearth's bicep swivels), but Windam seems more solid than both of them. Not dealbreakers for me, and probably easily fixable with whatever the cool kids use for joint tightening these days.
...also Windam and Rayearth both seem to have the same set of extra parts that look like they'd go toward a Combined Rayearth? But like, not nearly enough to complete it, and the non-gold ones of Windam's are in green rather than red. Not sure what those are about, especially since a Combined Rayearth was announced...