Since they are disbelievers and deserve eternal punishment, it is not unfair that they suffer in this life as well. Whoever disbelieves deserves eternal punishment and whoever dies a disbeliever is guaranteed eternal punishment.

They have suffering in this life and unending suffering in the next. Another thing to consider is that compared to the punishment in the next, this this life is in comparison Paradise. But the point still stands-they deserve eternal punishment so there is no injustice for the suffering they go through in this life.

As for the ones who don't receive much pain in this life, they are in for more of a surprise when they enter it, but ultimately this was just Allah's relief and favor on them in this life. Since they too deserve eternal punishment and are guaranteed eternal punishment upon dying as nonbelievers. In conclusion, every kaffir deserves eternal punishment so there is no injustice in the fact that some suffer to varying degrees in this life as well. Each of them ultimately is in punishment forever.

May Allah guide us.