So far what I've seen, and heard from a couple of "brella mains", the brella seems like a heavy support weapon. It doesn't seem like it can carry games or even attempt to do so.
The basic strategy I've seen on the brella is that you want to get up to their face, and alternate shooting and blocking to win gun fights. Therefore your chances of winning against short range shooters is pretty good. Your umbrella is also great at blocking specials, as you can negate splatdowns, ballers, and even inkjets with ease.
However, with a good blocking ability, there are some things you cannot protect yourself from, like blaster shots being able to tear you apart while you can't do anything back. Also, the heavy can shred through your umbrella and still kill you. Another problem with having no range and not too many ways of closing in distance is that you are extremely passive when it comes to combating snipers, jets, and even splattershot pros.
So, I'd say this is a very passive weapon, with the goal of trying to support your team. This weapon can't cover turf on its own, so it's nice the sub and special try to help that aspect. Still, you will be combated with multiple range issues and standoffs that are impossible for you to win on your own. But, having a splash wall almost always at the ready is an amazing ability that shouldn't be overlooked.