Some thoughts about Wattson:
Probably has a good but not godlike hitbox.
Her highly-visible, defensive abilities seem like they may just prolong engagements and draw third-partiers towards you.
You might hope to block off the third parties' invasion routes to deter them from coming at you and encourage them to flank your enemies instead. This would be pretty bigbrain.
Her Ult basically promotes cover to super-cover (impervious to nades, and it regenerates your shield). That's pretty neat, but the above issues make me figure she'll be like Caustic in being so niche/situational/challenging as to be largely irrelevant. It's interesting that she can place up to 3 instances of her ult. I wonder how long each ult lasts before it disappears. (Edit: I just watched a vid I hadn't seen before and saw that her ult is a "permanent structure until its destroyed". So yeah, you want to place it in cover, but it'll last forever.)
Also it's unclear how much her electric fences actually hurt, so perhaps if you're not actively guarding your fences then enemies might just go through them and tank the damage just fine.
All that being said, it does sound kind of fun to deliberately play with a strat of luring in third parties and then encouraging them to attack other teams instead of you. (Since to attack you they have to go through your fences, and you eat their nades with your ult.) I'd already thought of using that kind of strat with Caustic, but Wattson will be way better at it. She seems like a pretty much better, more team-oriented Caustic.
If she's actually a strong counter to (or at least defier of / exploiter of) the super aggro, third-party meta then that'll be interesting. A more cautious, defensive meta would be… interesting, for a bit. Then people would be like "this is boring."