Good post. I've asked the question on the forum several times in the past.
What is a free safety and what is a strong safety?
Typically, a free safety primarily supports the cornerbacks in defending against the pass. A strong safety is involved in pass defense, but is utilized very often to defend against the run. It doesn't even MATTER what defensive formation it is, 3-4, 4-3, etc.
What often gets me is when someone begins an argument which something like, "Well, Ed Reed can blah, blah, blah!" Well, gosh darn it. Ed Reed's a free safety and a Hall of Fame caliber one at that. You can't make that kind of comparison between Reed and Roy Williams because Roy Williams' strength isn't at free safety--even though he has played well at that position early in his career when Darren Woodson (a.k.a. STRONG safety) accompanied him in the defensive backfield.
His strength is at strong safety, but to gain the most dividends out of positioning a strong safety well, your cornerbacks and free safety have to do their jobs defending against the pass. If they do not, what choice does a defensive coordinator have BUT to position his strong safety back into more and more pass defensive alignments--which, in turn, takes away from his strengths and heightens his weaknesses.
I'm not defending Roy Williams. He has issues too, but if he's going to be criticized, his critics should at least know what the responsibilities of a strong safety (and a free safety) are first.
Truly, I blame the lack of basic understanding on too much video gaming. In the video game world, any safety can defend against the pass like Ed Reed. That's just does not what really happens on ANY level of real life football.