The ostwind is far better in the AI role then the brumbar. a brumbar can do a lot of damage in a single shot. but it needs to hit to do so while the ostwind will simply load another 37 mm round in its gun and keep firing. Also the brumbar rarely squad wipes unless units are clumped up.
Which is all the time.
By the way, it's way better microing it with attack ground onto a particular unit clump rather than hoping the AI of it gets a good target (frankly, it probably aims for the center every time), especially because of the long reload time. It still won't be
great in practice, but it's much better.