The is-2 had serious crew comfort issue,s. and it,s ammo consisted of 2 pieces so the average reload time was 2 per minute. The german army also tested the penetration power of the 122mm a-19 gun and proved it to be much less effective when the panther was standing at a 30 degree angle. also the barrel life of the gun itself was very short.
The best tank of the war if looked only at the tank itself without any logistical and industrial data taken into consideration is undoubtly the king tiger. The king tiger had the best frontal armor of any widly used ww2 tank(so no maus) and the 88mm kwk43 l/71 gun is considered the best gun ever put on a tank of ww2. it had atleast 3 times the rate of fire and a penetration of 203mm on 100m distance versus the is-2,s 175mm penetration at 100m distance. combine this with far better german optics and you get a much more reliable and superior gun. If the king tiger was used by an experienced tank crew with enough fuel at it,s disposal. it was the tank to figth with.
Much of the mechanical problems the king tiger had where caused by inexperienced crews. Ofcourse the king tiger had it,s limits like mobility issue,s(it still was damm fast for a 70 ton vehicle going at 38 kilomiters an hour on road. this speed is around the same of the is-2) and mechanical problems which would be an issue if the tank was handled by an inexperienced crew. The site achtung panzer even state,s that the overall reliability of the tiger 2 was better then that of the panther.
diffrent tank aces like kurt knispel never complained about the reliability of the tiger 2. I actually think no surviving german tiger tank driver ever said that there tanks where unreliable. while they are the ones who have driven those steel beasts. So is the tiger 2 that unreliable? i don,t think so really.