I 100% agree with the hardest to rank are OLs. My general rule was I trusted myself more with my own evaluations the further away a player plays from where the ball starts (so like you said C, OG, OT, DT, etc.). WR and DBs always seem much easier evaluations to me when I had to do them. Especially given it is relatively easier to plug in a HS WR/DB into the college level (or they are relatively less work to develop)
I can see why you may say QBs are easiest to evaluate, but I would also argue that given all the mental aspects, the small footwork details, technical aspects of the throw, and mental aspects again, it can be a very hit or miss on evaluations. Also there are multitude of different schools on how to teach and perform at the QB position. However, there is a loooooot of tape on QBs and a lot of people who know all that stuff very well.
NFHS is a great resource.
I think another thing that can help with evaluations is actually who you know in the business. If you are able to see private work outs and actually shoot the shit with a coach that knows the position better than you, it helps you understand what to look for and how to evaluate talent.
All this to say, I agree with caveats. Good points made.