The very basic game mechanics in FB and 40K are very similar, so of course they can be compared, and the obvious difference is that 40K has guns, whereas FB mostly revolves around close combat.
Haven't played AoS, so cannot say much about it. Yet even there, when you have the GW design principles and granulation of d6's, I can imagine AoS can be compared. Especially when both AoS and 40K are mass-skirmishes (no regimental rank-and-file units).
-Z