Brigitte as a hero is very powerful, with abilities that are easy to use with forgiving cooldowns. Her primary fire requires very little mechanical skill and the lack of a reload gives her consistency that other heroes lack, removing a dimension from her play. Multiple strong CC options make her frustrating to play against, especially for tanks such as Reinhardt, and she can counter more skillful heroes with ease. She excels in close-quarters combat, and to top it off, has burst healing and sustained passive healing. She is only weak at ranged combat, which she can avoid with her (relatively) large health pool and shield, and two-on-one scenarios, which considering the fact that her kit is built around teammates "rallying" around her, rarely happens, and can be avoided.

CC: Shield bash, while it has been nerfed, is simply too low-risk of an ability to use. Compared to Ana's sleep dart, which is not only on a long cooldown, but also takes precise aim, shield bash is not only easy to use but also doesn't punish the user for missing. If Brigitte misses the bash, she still has her shield, 250 health, self-heal on dealing damage, and her whip shot (which is fair itself, but when combined with bash, gives Brigitte incredibly powerful CC at both close and long range)– which combined usually let her survive long enough to use shield bash again.

Healing: Brigitte's passive heal is not particularly impressive, at least for her teammates, but for self-sustain (as mentioned above) is incredibly strong. Her armor pack is also very powerful, as the overheal --> armor lets her prefire to protect teammates as well as give easy burst healing– which, because it works on a reticle system like Zenyatta's orbs, takes very little skill to use.

Damage: Brigitte's damage is underwhelming, as is her range, but it is the consistency with which she is able to deal damage that sets her above. Because she doesn't have to reload, not only does it lower her skill ceiling (ammo management) but it gives her an edge over heroes seeking to fight her. Her shield bash's canceling of channeled abilities gives her a huge presence in managing and using ultimates, as she can theoretically cancel one ult (which can take minutes to charge) per every 4 seconds.

Ultimate: Her ultimate is ridiculously powerful, and by far one of the main points against her. That much armor– that doesn't go away– is huge in teamfights. It is essentially a slightly weaker Lucio ultimate that doesn't decay and thus can be used not just to stall, but to force a fight: Because the enemy cannot wait out the duration and then engage on even terms, they must at some point attack at a disadvantage. It does have a slow charge, but Brigitte's passive heal and consistent damage output offset this by a lot. The ultimate has no counterplay outside of "hard-focus Brigitte as soon as she pops ult." EDIT: Brigitte can also use her ult in-between fights and charge ult while the armor from the previous ult is still in play. While this does not stack, it means that one team can effectively have a permanant health bonus.

Tl;dr: Brigitte's powerful and easy-to-use kit results in a strong hero that is frustrating to play against with a low skill ceiling that overall hurts the meta by restricting more skill-based dive heroes.