He’s not much of a team orientated hero and he doesn’t synergise well with Mercy. There’s no barrier, and he moves erratically and is quick to break LOS so that’s often a suicide sentence if you’re silly enough to follow him into battle. He is not like Winston, who you can follow into the enemy lines and he’ll provide you a bubble and a quick escape by launching into the air, providing height which is an important resource for Mercy as height expands you LOS and allows you to quickly get to other hurt allies or to GA to the back line if you really need to get out of fire quickly.
Hammond is more like a Tracer, basically a hero that you need to ignore for the most part and allow him to go off and do his own thing until he comes rolling back to you begging for healing. Topping his health up is an arduous and time consuming task so I put Hammond near the bottom of my list of target prioritises. When a brawl is breaking out I’ll pretty much ignore his health completely and prioritise squishes and other tanks over him. He takes too long to heal up so there’s almost always a more worthwhile target to be healing.
Hammond can be very annoying and make you feel like it’s a 6v5. As with any hero, in the right hands he can get work done, but bad ones will take massive damage and then expect you to slowly heal them back up so they can roll out and repeat. This isn’t efficient for the team because every moment I spend slowly creeping his health back up isn’t spent on healing DPS/main tanks and damage boosting.
Hammond kind of performs a one man version of the Winston/Mercy dive as it creates space and chaos in the enemy line, but Hammond is for players who don’t/can’t work as a team and that’s often less effective.
The other way I see Hammond being used is like a flanker Tracer / Doomfist who will be off playing around on the other side of the map whilst the real battle is going on the point. He can often pretty much just be a pretty bauble tossing itself majestically across the map but not really providing any actual help in terms of killing power or protection for his allies, and you’re left wondering “What’s the point of that?” 
Ok, so onto the positives of all this!
These negatives can be positives as long as the Hammond is able to do work by himself. As with Tracers / Doomfists, the requirement is on him to prove his worth and justify his position in the team. It’s okay to not be aiding the team as long as you’re doing work by yourself.
Good Hammonds who can score kills, create mayhem and lose minimal life in the process by making clever use of mobility and his shield generation mechanic will provide enough advantage to his team and limit the healing down time required from his teams healers. Time spent healing Hammond should occur between fights ideally, rather than during them, and it’s on the Hammond to ensure that the Mercy doesn’t have to switch to baby sit him too often.
In short, a good Hammond does work without needing baby sitting. A bad Hammond contributes nothing useful to the team and takes enormous damage, feeding the enemy and taking time off of his healers. I feel that more team orientated heroes such as Winston can generally be more effective on a push, but there’s always exceptions. The same rules apply to Doomfist and Tracer. You can get some epic Doomfists, Hammonds and Tracers, but also a LOT of bad ones.