Also the damage type versatility is not to be undervalued. Resistance to piecing damage sucks for a rogue otherwise.
Because they have to switch to their Scimitar?
I suspect the OP forgot that rogues get cunning action, and how not using an off-hand weapon frees up its use.
GFB and BB's problem is not necessarily with its damage (which is still a bit high considering it has a rider and it stacks on top of weapon damage), it is its weird progression. It falls into this weird niche where generally only characters with one attack like it; for some characters, the cantrips are a redundancy, but are no-brainers for other characters. That's bad design.
It's best for (I'd even say essential to take full advantage of) Eldritch Knights. It is your best attack at levels 1-4, excellent at levels 7-10, still the best choice at 11-16th level when used against multiple targets, and at levels 17-19 it is simply the best option. The only levels where it isn't much use to an Eldritch Knight are 5-6, and 20 (and even then it can rock against multiple targets). The rest of the time, if your Eldritch Knight isn't using it they are doing it wrong.