While I generally agree about the BT, let's examine it a bit closer, assuming RoPen + , vs unruned Cannon (let's assume we only care about hitting 1 model or at least the front rank of a unit)
Same cost, same range
Cannons hit like 50% or something without forging (10-inch lead). Has chance of blowing up.
BT with accuracy hits 50% at long range and 66% at(assuming no cover/skirmish).
Cannon is S10, multi-wound(d6) and can pierce ranks
BT is S7, multi-wound(d3), no AS and can pierce ranks
So to summarise, with the BT you hit as much or more often, have no chance of blowing up, but are likely to do less damage (although still significant). Against primarily T5 and lower models, the BT is often going to be the superior choice. Against T6, it's still pretty good, and against T7 it starts to lose efficacy. I would seriously consider running this BT over an unruned cannon. If the points allowed, I would never run it over a cannon with forging. In larger games, I would probably run this over a third cannon, as having 2 cannons allows the BT to deal with lower-T threats, and getting forging on the third cannon starts becoming costly without resorting to double flaming shenanigans.
An extra RoPen helps secure that critical to-wound roll, too. Another option for increasing effectiveness at shooting fliers is Flakkson's rune of seeking.
TLDR; Possibly better than an un-runed cannon. Take instead of a 3rd cannon in larger lists.
