Whats the difference between rengar's q and nasus's being able to hit towers?
Rengar has invisibility to escape. Rengar built attack damage, so his normal basic attacks deals significant damage to the tower. Rengar got attack speed from his Ferocity-Q to push faster.
Nasus doesn't have a way to escape enemies aside from Withering them and running. If he's caught by more than one enemy, he'll have to try and fight or hope they don't have any CC to chase/kite him. He doesn't typically build damage or attack speed (with the exception of Trinity Force), so his normal autoattacks don't really deal a lot of damage to towers.
Why is nasus's q unable to be blocked by any ability that blocks auto attacks?
Pantheon's passive blocks Q. Blind negates damage dealt by Q. Parried by Fiora's W. Spellshield's block Q.
Since Nasus doesn't typically build Armor Penetration, he only really deals significant damage to champions which don't build defenses. Against bruisers or tanks, Nasus deals significantly less damage unless they have built primarily damage.
Against attack speed dependent champions, his Wither can help equalize a lot of fights. It's generally recommended to build a Quicksilver Sash at some point to remove Wither (1250g later in the game isn't that significant).