While it does have less engine power than a regular IS, the regular IS is already one of the more mobile tier 7 heavies in the game. I have played about 5 games in the IS-2S so far and I have not gotten into a situation where I feel I could have gotten out alive if I were driving my normal IS
If people are expecting more out of a premium tank, this becomes an issue of premium powercreep, which is a seperate discussion in itself.
This tank is a prime example of an old-meta premium, ie, it is objectively worse than its tech tree counterpart, but makes up for its shortcomings by having a pretty good credit earning multiplier. The thing is that while it is worse than a normal IS, the IS is a pretty good tank to begin with, and having less engine power is a small issue when you get practically the same gun performance, while earning more credits.
If people are saying this tank is a lazy copypasta of a normal tank, they would have said the same about the black series of tanks, the fact that at least 3 nations have their own flavor of panther at tier 6, and how the KV-2 with the warhammer 40k skin is also a separate vehicle from the normal KV-2. Don't forget how the Pz IV Schmalturm was added after the normal Pz IV lost its Schmalturm and the VK3001H was moved from tier 6 to 5, losing its Schmalturm as well.
If people dont want to pay for a slightly downtuned copypasta of a pretty good tech tree tank, they could have gotten it for free if they started the event on day 1. I got mine for free, using only the 15x boosters from the daily missions. I used the 15x tokens as I got them so I could unlock tier 3 and farm at that tier using the 5x daily boosts as soon as possible, and when Berlin was finally released I was only 6k points from getting the tank, which was 3 Berlin games.
27000ants, Asia server, if you want to check my IS stats (not great, not terrible)