I haven't run across a Tiger in a PBEM (yet), and I certainly wouldn't buy one unless an opponent specifically wanted the challenge of taking one or more out, but FWIW, the HEAT shell fired by a Priest can destroy a Tiger. They're not a match, obviously, but the Tiger isn't unkillable by Allied armour.
Unkillable,definitely no. You can kill the beasts from close range with bazookas/Shermans,assuming you get enough side/rear shoots. You can also kill it with 57mm AT guns,again,from the sides/rear. In a recent game i took out a Tiger with 155mm arty.
The issue is,at least in CMFI QBs, that,while not unkillable,Tigers will seriously tip the balance in favor of the axis player.
I wouldn't argue with them tipping the balance. Anyone who wanted to take one or more ought to signal their intention and even offer their opponent a points bonus, which wouldn't be unrealistic, since in the typical CM size engagement, the Allies (in particular) would be likely to have a fairly shrewd idea what kind of units they were up against.
Personally, if it came down to both players having fun, or myself winning, a fun game would win out every time.