I do agree to a lot of your comments, when I made this post I was thinking especially about AT guns, omnipurpose guns / fire support guns are really usefull but I was more in a situation where I only got at my disposal infantry available (with their support equippement) whereas my opponent would have access to a large panel of armored vehicles.
So would I be able to bring my AT gun close enough, good LOS and set it up fast enough to get a shot to the ennemy armor ?
For this I'd think that the size and nature of the map is critic : you could bring an AT gun to use if you can set it up in a way where the gun is far enough and have an appropriate LOS where you need to attack.
But even in this "perfect" case scenario you'd still be vulnerable to ennemy fire (guns, mgs) and of course a well aimed mortar barrage would put an end to this.
However I'm pretty sure that if you try to set up an AT gun on an uneven map and/or too close to the ennemy your try will end well before you could say.
So if I'm on a situation where I couldn't rely on armor to deal with potential armor, even if I put the map selection on random, I wouldn't spend ressources on canons, even if they're cheap, but I would make sure to have a lot of panzerschreks/faust/PIAT/Bazookas and on regular artillery(off map) not to waste my time on a tricky canon set up, the odds are too unfavorable for it to be worth it IMHO.