D.Va in the current meta is like Winston if Winston was anything more than a surprise pick (again, in the current meta). Hopefully Blizz finds a way to make both dive tanks viable soon.
The health/armor switch should help a lot in terms of bringing D.Va down, but I think what I'd like to see happen for Winston is for direct hits from his jump pack to do more damage, up from 50 to 75-100 or so. It'd make him a more reliable threat against Tracer and would avoid the problems inherent in buffing his barrier or gun too much.
I'm never really sure what I'm supposed to be doing as Winston.
He seems squishy for a tank, and his shield is so wimpy. His ult doesn't even do that much damage.
Surviving as Winston is all about positioning and effectively managing his staggered cooldowns. Always try to estimate how much damage you're jumping into and make sure that your bubble will last the 6 seconds it takes to refresh your cooldown and jump out. The bubble only has 600 health, so it's going to get focused through immediately if you dive straight into the center of the enemy when no one else on your team has engaged. Instead, try to jump in on backline healers, elevated range characters (Soldier, McCree, Hanzo), and any lone players who are trickling in from spawn.
When your bubble does start going down and you jump out, keep in mind that your jump will refresh long before your bubble does. It can be tempting to jump right back in after six seconds, but that leaves the enemy team with 7+ seconds to shoot at your enormous hitbox for free and you'll probably just die. Use that jump to go find a healthpack instead and then push in when your bubble's back.
Lastly, avoid Reaper and Roadhog like the plague and only push in on Zenyatta if you're confident you can approach and place your bubble before he discords you.
And his ult is trash in terms of damage potential, yeah, unless you're near a pit. Like the rest of his kit, it's mostly about distracting the enemy and breaking apart their formation so your DPS can clean up. As with the rest of his kit though, it's extremely weak if your DPS is unable to push in and take advantage of the openings you create.