As a guy who has toppd and bottomed. Getting hard and/or staying hard as a bottom isn't a big issue for me. I usually don't cum anyway so if Im hard or not doesn't matter. I have been hard getting fucked and its usually because the top has seriously turned me on somehow or I am really into him.
As a top, if the bottom stays hard while fucking him it is a definit sign he's enjoying it but if he isn't hard you can still tell if he's enjoying.
My biggest issue is actually shooting a load. I have always been this way but I never cum with anyone new. It takes me a long time to shoot, I can shoot in minutes jerking off. Go figure. I still enjoy the fuck though either top or bottom.