The problem with heavy tanks in general KT, Tiger and IS2 included (less so the IS2, to be fair), is that they don't have any real counters, or drawbacks.
Once they become unlocked, there is very rarely (if ever) a situation where getting one is not the right decision. They are simply better than your other units, which effectively reduces your range of choice.
Allow me to quote the balance master himself,

:
If a certain set of decisions are the BEST decisions then there are in fact no decisions. Make it difficult for me to determine what course is the best course and you stimulate my mind.
New units should enable the player to play differently. They should open up new options. But they should never replace the usefulness of other units!
Limiting the availability of these tanks to 1, doesn't erase the underlying bad design.