As Professor Boris Karloff once said to a guest in a horror film-
"Perhaps you'd care to step into my laboratory for a moment, I'm conducting an experiment that may interest you"
Here's a screenshot from an experiment I'm currently conducting in the editor, trundling an assortment of bunker-killers and tanks up to the Fort and Strongpoint to see what they can do, and monitoring the pre-combat 'Extended Combat Prediction' figure with 'CTRL-click', and likewise monitoring the post-combat figures with key 'L'.
All I can conclude so far is that the "bunker-killing" AFV's big trump card is that
the Fort/Strongpoint can't fire back at them even though they're adjacent to it, because presumably they're lobbing their low-velocity shells on a rainbow trajectory over intervening walls, trees and buildings etc from where the F/S can't see them.
Like guys have said, their shells tend to suppress rather than kill, so tactically I envisage them shelling the F/S's to suppress them while an infantry unit or tank moves up in the same turn to finish them off in a neat little combined ops move while they're still dazed..
BK's also get a +5 bonus to their hard attack strength against F/S's which further helps to rattle their brains.
(And for the record I think the term "bunker-suppressor" would have been a better label to use in the game than "bunker-killer")
PS- I also used a few tanks in my test and although they certainly have a higher chance of killing some F/S occupants outright (presumably because their high-vel guns have the accuracy to put shots straight through the slit), there's always the risk that the unsuppressed contingent of the F/S occupants might fire back, so it all depends whether we're willing to accept possible tank losses.
