I always thought of the IS series as a technological dead end with the IS IIm being the pinnacle of its development, much as the US M103 and British Conqueror were technological dead ends. By Soviet standards, the IS III was produced in just tiny numbers (a few hundred).
Its flaws included:
*Problems with weakness of the hull resulting in split welds in the early production models.
*The tiny ammunition supply of 28 rounds. This is compounded by its two-part ammunition and low rate of fire.
*The vehicle is very cramped making crew efficency difficult. This and the ammunition supply were addressed in the follow-on T-10 (IS IV) by lengthening the vehicle to a degree.
*The lack of a commander's cuploa and the small number of vision devices provided mean that the vehicle is almost blind in a mobile battle.
*The forward opening hatches for the commander and gunner are a perpetuated mistake. They make it more difficult for the commander to operate in an unbuttoned state in combat without unnecessarily exposing himself to fire.
*The slack track system is prone to failure.

The usually cited beginings of the MBT are found in the US M26, British Centurian, or Soviet T44 /T54.