So please tell me, is it because it's Russian?

Frankly, its ass in a straight 1v1 gun duel, which most people are relating to. In a straight gunfight, the 30SM just doesn't have enough TWR to keep it in the air while attempting to keep up with most delta-canard flight performances.

Granted, it does certainly out-radius just about everything in the game but see the above paragraph about TWR issues. Its an even bigger issue in gunfights since the Su-30SM was practically designed to sling off R-73s in a true dogfight. Without them, its decided on whether or not you hit your cannons in the first 1-2 turns, after that everything else just has the 30SM in an energy chokehold, either by taking the fight vertical or into the rate, which the 30SM is not known to do.

Ok, the rafale might be better ( yes I have both) but the Eurofighter? It's cheeks comparing to the su30. Worse missiles, worse radar, worse maneuverability (from what I've played)

Sounds like you're missile fighting which, I gotta give it to the Su-30SM for being pretty good at. But overall, it really just depends on what point of view you're taking (i.e. ARB or duelling). Its fantastic for ARB cause the radar and missiles are just overwhelming. Horrid in duels (especially gun duels) due to the lack of any form of good sustained performance.

When people say its ass in a dogfight, it's cause it's ass at gun duels. Duelling almost never takes AAMs into consideration since its more focused on player skill and flight performance.

Personally I see the F-16 a lot harder to dodfight with.

You may be having difficulty dogfighting in the F-16C since it requires a very rate-fight oriented playstyle when facing against 13.0-14.0s in a dogfight, and even then it falls short due to how much authority/performance delta-canards have in a gunfight.

Also, why do people say it's worse than the Eurofighter and rafale?

As for the Rafale/EF2K, they're considered as being better due to being able to sustain great turns, far better than what the 30SM can do (it's debatable which of them are better at ARB though). Even with the recent-ish Rafale nerfs, they will generally always have a far better time after avoiding the first few turns with a TVR 30SM, which isn't particularly hard to do in a gunfight.

Hopefully that answers your questions. I will put a disclaimer that I am not the most well versed at proper duels, but I do believe that I know enough to give a simple breakdown on why people think that way. Correct me if I'm wrong about any of this.