This is a suprisingly good and well thought out addition. One of my players really likes mechs, etc., and is always asking for modern or futuristic weapons. This is going to blow his mind.
We're going to start SKT soon, and I plan to replace the fire giant's forges with a crashed futuristic ship, a la Transformer's Ark. The ship will be a mech workshop, run by a fire giant with gnone slaves/scientists. Liberate the slaves and gain control of the workshop. Players will be able to pilot a heavy mech to fight giants and maybe find a light wheeled vehicle for travel.
The additional character features will take some playtesting before I can offer an opinion. That said, the bard features have pleasantly cheesy names. No "for whom the bell tolls?" or "aces high"? The wizard spellweaving features will be really interesting. Seems like a gnome wizard of the school of automation would make a great pilot.
A diagram showing the characteristic hierarchy of an ATUM would be helpful. I.e. chassis -> engines, fuel tanks, manipulators -> hardpoints. It's confusing to keep all the new terms and dependencies straight. It would also be helpful to break into separate player and DM sections, so I can share the new rules with players without giving away all the options/mechanics.
[5 of 5 Stars!]