Those three that went to Portugal became notorious back home for their lack of dedication according to the russian\ukrainian press that followed their exploits there.
You are entirely right and well informed. They were coached by Sven Goran first and after by Toni, if you knew how many times he had to put them without playing regarding their nightclub adventures.
But its fair to say Yuran and Kulkov had some good performances with them, particularly at European big games. Mostovoi problem was he wanted to play more, he didn't reacted well to be benched, Rui Costa was ahead of him. Now I remember Benfica loaned him to Strasbourg before he joined Celta Vigo.
I never rated Yuran too highly, the guy was a proper scumbag too with his repeated dangerous drink driving convictions, but Kulkov was a very classy CB\DM that could have had a better career.
Another player from the former Soviet Union who was a hot prospect at Sporting who unfortunately got a sad ending of carrer associated with drink driving was Serguei Cherbakov, he was signed by Bobby Robson in 1992 from Shaktar, he was starting to explode on a atractive team we had at the time under Bobby on 93 with Paulo Sousa, Figo, Balakov, Valckx, Juskowiak and so on.
Unfortunately our stupid President Sousa Cintra at the time fired Bobby because we were eliminated by Casino Salzburg on the old UEFA Cup, at the time we were leading the league, so he fired him and at the end of the season we lost.
On the week they fired Robson the entire squad went to a goodbye dinner with him and Mourinho, fair to say Cherba after the dinner probably went to some drinking session, the accident fractured Scherbakov's skull and his spinal column in three places.
You probably know this sad history
https://en.wikipedia.org/wiki/Serhiy_Scherbakov
Fair to say it was a different context when those players left the Soviet Union to play on Western Europe, still think even today, be it Russian or Ukrainians, they still struggle, just look at Konoplyanka or Yarmolenko.