Visas in Serbia: Personal experience
First from Albania went into Kosovo, where visas are not needed. There were no problems on the border. Inbound stamps set, and also rumbled a little in things (the only time for the entire trip to the Balkans). Next, went to Montenegro and went to Serbia from Croatia. Serbs did not respond in any way to Kosovo stamps, although they saw them for sure (passport was cleared).
I went to Serbia without a visa and without vouchers (citizenship of the our Federation), and there were no reverse tickets. Friend with Ukrainian citizenship went with a visa. In both cases there were no problems.