But as far as AT-ATS go, they've been impractical since 1980. Why have legs when you can hover/or fly? No legs to trip!
I only have a problem with the AT-ACTs, and not much else and for me the problem with them is that they seem pointless from logic/military standpoint, why would you spend the time and resources in building an AT-AT like cargo hauler when you could build something much simpler to do the same. It would be like the US taking an M1 Abrams chassis and converting it to haul cargo when we have a fleet of trucks to do it for much less? It just smacks too much of coming up with a new design for the sake of a new design, not that I'm opposed to new designs but they should make some sense.
As for the U-Wing, it makes perfect sense why we don't see it anywhere in the OT, it's because there's no need for it at any time during the OT. Think of it, in both ANH and Empire the Rebels were on the defensive, so no need for a light troop transport there, they have a base to protect. In Jedi, while they do go on the offensive it's also a covert mission that required sneaking in in a stolen Imperial shuttle, there's no way they could have pulled off the Endor raid in a U-Wing given that it doesn't seem to be an Imperial ship.