I can only speak about Kotobukiya’s kits, haven’t touched Tomy’s. The engineering precision isn’t up to Bandai’s lofty standards. You’ll run into plenty of jank, such as pieces barely fitting together, requiring uncomfortable amounts of force and large gates in unfortunate locations. For example, the Geno Saurer here had large gates on the inside of a curve on the forearm armor pieces, the purple ribs along the spine are held in place by a handful of tiny pegs and material tension, and I had a hard time getting some of the polycaps in their sockets. The manuals throw a lot of info at you at once - each segment of the tail consists of nearly 20 pieces and the assembly instruction was a single panel per segment. Plastic cement is recommended, if not downright necessary in places. Some kits that come with prominent pieces painted on the runner, such as the Shadow Fox and its gold, will require painting, since nothing is undergated.

That said, they’re well worth the effort in my opinion. They certainly take more time and care than your average MG, but the end result will absolutely stand out in almost any display. Or maybe I’m biased, since I got into the hobby with a HMM kit