European registration and airworthiness issues:
In the 21 years that we have been dealing in Eastern Bloc aircraft, we have encountered a variety of bureaucratic obstacles to their operation.
It should be noted that aircraft are either fully Certificated (as Cessna/Piper) or not. Those that are not cover a wide variety of aircraft, including many of these Russian types, but also historic aircraft; homebuilt; 'Warbirds' etc etc.
So in some countries, obtaining 'Experimental' Certification is relatively simple (US/Australia etc), but in others more difficult and bureaucratic. So within Europe, Yak 18T, Yak-54, Sukhoi Su-29 and 31 have obtained EASA Restricted Certification. In practice this means very few restrictions, although some exist in terms of commercial operations. Having said that there is no problem to do airshows; aerobatic instruction etc.
Yak-55 and Sukhoi Su-26 have been given EASA Permits to Fly, which, in practical terms, does not make a lot of difference from the EASA Restricted Certificates of Airworthiness for the other aircraft.
All other planes fall into the European 'Annex II', which essentially means that their airworthiness is controlled on a National basis. Unfortunately there is no agreed uniformity between different countries as to how this is managed; what restrictions there might be.
the last 15 years or so, many European based Yaks and Sukhois have been using
Russian registrations from FLA, the Russian Federation of Aviation Amateurs.
This was accepted by most European national authorities, but first the UK CAA decided that FLA did not have the authority to issue airworthiness documentation, and although the French DGAC for a while allowed aircraft registered with FLA to fly in France, that permission has now been stopped, and the DGAC are looking at ways of allowing aircraft to fly with a formal French restricted airworthiness documentation. The LBA in Germany are doing the same.