Lead free soldering was a problem, while people learned how to use it. For those who have now had years to learn how to design and manufacture with it, it is no longer problematic at all. Where I'm currently working everything we do is RoHS compliant and we have no issues with that.
For those who are still learning, with proper quality control it should only be a yield problem, not a reliability problem.
I would not be the least bit worried about RoHS in avionics!