you could classify the exc somewhere in between the smr/sxf and the LC4 range but they still are closer related to the more high end machines.
the new exc motor still needs to proove itself, since I own one myself I hope it'll all work out :lol:
the old motors are reliable if maintained properly. they might not be super sensitive but they still require rather frequent maintenance. on the old RFS motor you're looking at oil changes every 15 hours and valve checks every 30 hours. rebuilds should be around 250 hours or more depending how you run and treat the bike. mine ran pretty much flawlessly for nearly 400 hours, did a total of 15000 miles on my '05 450 exc.
the new 450 and 530 still have me guessing, since the valves work with shims valve adjustments might not be as frequent compared to the older bikes but I can't put a numbre of hours on it yet. as far as oil changes go, I'll prolly change the engine oil every 10 hours and the tranny oil every 20 hours. I'm almost done running her in so I'll check what the oil looks like when I drain it.