smoke trail a problem ?...well its common in Russian engines...when Pak use RD33 engine for Thunder , that was much smoke...now they use RD93, it has less smoke problem..
As far as training scenario, Its easy for Pak engineer to look inside the machine, rather then breaking a warranty seal and pay hefty cost of re-assembling .Chinese provide proper tools of maintenance, plus changes in machines according to requirement. Very few bugs attached with that. But for any machines, European attached to many rights and warranty. No doubt, Chinese radars are not proven product yet. But we are making statement without even knowing the product. We can't mind set that Chinese defence product are inferior. After all PAF pilots successfully used Chinese Migs in aircombat against India.