It looks like a done deal, but what's the reason for so much doubts? In a semi-related question, will the S-400 deal go through?
there are two reasons
the first one is the deal has taken really too long, it has taken around 5 years from showing interest to sign the deal and talking about deliveries with a date.
Chinese interest started in 2009, the first signing of a
memorandum of understanding was around 2012, the contract was signed in 2015, but even the CEO of Rostec in 2016 was saying that both nations needed to ratify the deal for the deal to go through, it was only in September 2016 when they announced the Deliveries are set for this year end.
The second reason is copy rights, Russia got very angry at China for scrapping the deal of the Su-27 license, the original deal was China was going to build the fuselage, Russia the engines and related subsystems, China did not fulfill their part of the deal. and the J-11 started production with domestic content, by 2013 too Russia and China were talking about copy rights in specific Sukhoi`s license of Su-27.
The deal was smoothed, today even Sukhoi`s website says Su-27 production is going on in China, however Russian officials when asked said the best way to beat the competition was making the original better than the copy, Su-35 supercruises and has advanced TVC nozzles, no Chinese Su-27 can do that, further more Irbis has a range of 400 km for a MiG-21 RCS size target.While people say Irbis is PESA, the radar is not oudated, it probably can detect a F-35 at 100 km some people say, plus the Chinese version is downgraded.
Today Russia has type 30 engine, if you see that type 30 has such advances over 117s in the same way Al-31 had over Al-21 used on Su-24.Type 30 has more thrust, reports said around 16 tonnes, (some reports say 17 tonnes) better SFC, and it is lighter, it is a true 5th generation engine.
117S is more an equivalent of the engines fitted to Eurofighter, while type 30 an equivalent to f-119 fitted to F-22