First impression, it is kind of small.
Since Russia does not have a stealth plane, this is only a counter to the Chinese J-20. Then we have to consider it will be 20 to 30 years before this new fighter is finally in service. After their maiden flight, the F-22 took about 7 years before entering service, and the J-20 original prototype flew in 2011 and was put into service this year.
A stealth fighter like this, is not an export item. They will not be selling this to India or Taiwan, or Vietnam, or somewhere in Africa.
The small means limited range, it would still need air tankers to refuel. If this is going to fight China, then those tankers will not survive the current Chinese inventory of air-to-air missiles. No tankers, means this plane will not even arrive for the fight.
There lies the problems for this project.
Developmental time, that is really long. Maybe too long. 20-30 years.
Needs lots of money which no one seems to have. Even the Americans wanted their allies on board for the F-35 development because it was expensive. The Russians needed India to finance the Su-57, since they pulled out, that was that for that plane. Only the J-20 was financed and built in house.
Then the strategic necessity of this fighter needs to be asked. It can own the skies, but will Russia S-500 own it? 25 years later, it could be S-600 or S-700.
It's a big leap of faith this project.