Can I use 8 x 8" sheets on the Nomad?
Yes. Workholding is a bit tricky at that size but the masking tape/superglue method should be OK given the surface area.
Can the nomad cut .25 aluminum and .50 ABS?
Yes - if you are whittling down a most of a 8"x 8"x 0.25" aluminium block it will take a very significant amount of time and patience. If speed of cutting is one of your requirements the Nomad or just about any other “desktop” style mill is probably not the thing.
Does the Nomad hold up after daily use? or do you constantly need to adjust, fix, calibrate etc.?
As far as I can tell, one of the reasons there are less Nomad posts here than Shapeoko posts is Nomad tends to “just work” and the adjusting, fixing, calibrating is a minimum. If your accuracy requirements are very tight - a few 0.001"s - you will spend the normal amount of time fussing about.
Will it hold up?
If probably relates to how hard you run, but (again forum-wise as I haven’t had any problems) anecdotally Nomads have been run in day to day metal cutting operations and held up well.
Hope that helps!