Welcome to the boards Calichica.I agree with your assessment 100%.
Marty is a one trick pony. He didn't listen to what INXS wrote with their song choice for him - that they wanted him to lighten up and not be so serious. I thought his version was pretty cool, but they didn't give him that song because they wanted to hear grunge again. I thought they would have said as much.
With Mig, I don't think it's so much that he plays it safe, as that he's too sweet. He has no edge.
Ty has a beautiful voice, but I agree his performances leave me wanting.
And J.D. wants this more than anyone. It all goes back to what Dave Navaro said: the others are auditioning for A band, J.D. is auditioning for THIS band. The others are looking for a stepping stone for their own careers. J.D. will stay with this band as long as they'll have him.