As said by Lemonjuice and I've used it a few times, it can actually be decent vs a lot of infantry next to your medic truck or just a general protected position up field, i.e. keep it behind a hedge and use range. Can get 15-20 kills/game (~600% efficient), but it depends on the strategy and your opponent. Best vs rifle spammer/blobber.
if you're having trouble with B4, go spec ops for flares and make sure your partner has a doctrine with some off map that can destroy it.
also you can often just intimidate blobs into retreating without actually firing. for example aim and actually press fire towards the blob, doesn't matter if aim is a tiny bit off and watch them retreat. then halt fire before it fires unless it will actually hit them right on.