This is probably the worst thing in all this madness - the unpredictability. In the past if you wanted to fly, you bought a ticket and most likely you arrived to your destination. The exception could've been perhaps a crazy unexpected 2-hour traffic jam on the way to the airport...thanks for the suggestion, I'm thinking about Belgrade too, seems they offer vaccines to foreigners. that's such an annoyance, how can they change and start enforcing a new rule within like 5-6 days, AND all of this over the new year and weekend holidays.... feels like drunk monkeys are in charge over there... I'm fine with any regulation, but not for it to be signed a few days before it gets enforced (especially during holidays)...
However now EVERYTHING is unexpected. You must check rules to leave, rules set by the airline, rules set by country A, rules set by transit country B, rules set by country C, rules set by a local government or a certain city. If you want to make a trip with two or three legs, the probability of you arriving without any problems and any delays to do business or to go on vacation decreases drastically.
And the worst thing is that even if you do everything right, you are not guaranteed to be cleared. Because the rules can change and do change.
I totally remember that Montenegro (in the summer of 2021) was one of the most relaxed and free countries in Europe and in the world. You could arrive totally free, no vaccination required, no PCR test required, no arrival form required, no questions asked. Just show your passport and you're free to go. Not a lot of people wearing masks, noone annoying you. Now it is the exact opposite and crazy restrictions within the span of a few days. Well good luck, I thought 25 % of your GDP comes from tourism. Perhaps you already found some better activities, dear Montenegro.
Similar crazy changes were also in Romania, Netherlands... There are some countries going the other way and cancelling restrictions, like Mexico and El Salvador. Now maybe Spain or at least part of Spain? That's nice but they are still more the exception than the rule.