As someone mentioned before, the Su-27 and it's varients are beautiful and powerful aircraft...as an effective fighter, it's still an unknown for various reasons mentioned already. There are also a few other factors working against operators of said aircraft.
1) Training. Most operators can not simply afford to give the pilots the training hours and scenarios needed to stay proficient in their aircraft up to the standard of First World western eqivalents. As a part time reservist, I get 12 hours a month in a trainer aircraft. That's more than some countries give their pilots in 6 months. And I'm just a part timer. This makes the next problem worse...
2) Ergonomics. Even in modern Russian aircraft with glass, their general ergonomics are right out of the 1970s. As a human factors major, I tend to end up looking at everything from a human interface angle. When things are going great, you can get by with twitchy layouts. Not so when the sierra hits the fan. That's why 'sensor fusion' is such a big deal...the idea of being able to consolidate the various inputs an aircraft gets and sort it in one easy to interpret method for the pilot. I can't emphasize how important this is...and how it's only going to get more critical as more sensors are integrated into the battlespace. If your pilot's head explodes because he can't integrate all the info he's getting, it's all over...
FastCargo