I wonder about this too, having been weight training for many years. I feel that weight training helps me with strength (obviously), it does nothing re. flexibility, and it can be a hindrance re. muscle relaxation and ki extension.
My learned tendency is to muscle through my techniques and to tense up during a dramatic ukemi. That is my big aikido issue, lately, and I think weight training has helped condition me, at a deep level, to associate power with muscle strength.
So that is what I am trying to unlearn, in aikido. I am still lifting, and I am trying to get a grip on power through ki extension and not muscle strength.