I also find it really weird how often people will move out of my shield to fire at the enemy, even though they can shoot through it from safely behind it. Before it was mostly a visibility issue (the shield used to be more opaque), but ever since they adjusted the graphic to be much more clear, people still seem to go either side of my shield rather then behind it. It was actually kind of weird when during one Attack game a WM actually hugged my shield really well, since I am not used to it, lol.
As for Mercy her gun can pack a punch when you need it too, as it's similar to D.Va out of her mech, but switching to the gun means giving up the staff for awhile, which means it only are ever going to use it in circumstances you are more likely to get overwhelmed. (if even one other play is around you it's usually better to heal them).
BTW I am a bit sad, first look box I open gives me a legendary skin I really wanted, but I know after the 9th it will be gone.