All of these positions have 11 attributes/skills that they need to be good at. However, I think FS/SS are the hardest because you can specialize a HB and still be successful while it takes time for a FS/SS.
Honestly the lines are some of the easiest besides the kickers to build. O-line you need strength, blocking, and stamina to start. then move to agility and vision. then speed and confidence.
My center is level 11 and he's green in his blocking column.
DT is the one position where at a low level you used to be able to completely own any offensive lineman in you built him right but now all the linemen are getting stronger...