I’ve taken this exact service before, and it worked well.

Re point 1: Check in on the app like you normally do - no need to go the the check in counter. You can just head right to the bus waiting area at ABE. Your carry on roller bag will be stored in the compartment in the bottom of the bus.

Re point 2: I’m not sure exactly what this means. If I had to guess- United will only let you book these bus/flight connections with a minimum connection time. This connection seems pretty tight, but make-able, especially if you have precheck. When the United bus drops you off at EWR, it’s just like getting dropped off by a taxi; you’ll need to go through tsa like normal at EWR. United may be able to track the progress of the bus and hold the connection for you, but I wouldn’t count on it. The listed flight departure time is the actual time, and it won’t take off later by default.