I'll chime in with some feedback on this. Since I only played open a couple of times, I'm an expert now on NPC pirates.

NPC's don't present any real risk at present outside of the big bounty hunting missions. NPC pirates are easy to run away from after submission. If you stay and fight, I find some of them challenging enough.
Here's an observation though. A large portion of this games player base is significantly older than average, and a lot of us are drawn to solo and pve. I know I can't compete with a competent 19 year old's reflexes. The United States Navy, where I served, won't even let you become a pilot if you're over 26. Just as surely as I know that at my age I couldn't flight against a 25 year old me, I wouldn't want to dog fight against an NPC who dog fights like the 25 year old me. IMHO, solo cannot be as challenging as open, or there is just no point in playing, at all.