I think that some resistance can be one way of practicing aikido. But it is important not to resist the particular technique practiced at the moment - because that is almost pointless, and an aiki mood would really be for tori to immediately switch to another technique.
But uke can be sort of generally resistant - not just yield to whatever tori is doing. Still, this kind of practice can easily lead to nobody learning anything at all, so it has to be done with moderation.
Another way of polishing one's techniques is in kaeshiwaza
, countering techniques. A properly done aikido technique should really not be possible to counter - but that's easier said than done