In Germany, it was common practice to get refused into CAS, when some glider sectors inside class C or D were activated. This was quite annoying, especially during the weekends. Since this strange rule has been lifted somewhere in 2018, transits through CAS are almost always possible in Germany, except for heavily congested areas like Frankfurt as @UdoR said. Nonetheless, even Frankfurt CTR crossings are often permitted, as long as you present yourself with confident radio work outside rush hours.
Other countries are even easier for CAS transits, like Scandinavian countries, France, Austria, or Croatia. Even Italy is actually very accommodating, they even let you fly sometimes VFR in class A. Switzerland depends on which type of CAS: While CTR’s are generally never a problem, the Zurich and Geneva TMA is a kind of taboo for GA, as if it was some kind of class A just for “big birds”. The Netherlands is also quite alright in my opinion when it comes to CTR crossings. It’s just the annoying Dutch class A starting from 1.500 ft in the western part of the country.
In the end, it seems to me that only the UK is a real tuff country for CAS transits. Almost all other European countries are quite relaxed. Some more than others of course, but still…
Frans wrote:
Nonetheless, even Frankfurt CTR crossings are often permitted, as long as you present yourself with confident radio work outside rush hours.
Absolutely – due to the routing requirements, altitudes and potential holds, however, this is an option for “doing something special” but rather not so for “taking a shortcut when flying from A to B”. For all practical purposes, crossing the Frankfurt CTR takes longer than flying around it…
That’s true, but not always 
If I fly towards Egelsbach (EDFE), the crossing N(ovember) to L(ima) is clearly the shortest way. I’ve never been refused inbound EDFE, just a few times outbound. Yes, they let you sometimes fly one or two orbits north of the field (EDDF), but most of the times, I can pass directly overhead the motorway (BAB5) or over the threshold 25C/L.
Do not forget:
https://applications.caa.co.uk/CAAPortal/servlet/SmartForm.html?formCode=fcs1521v2
when refused access
In the end, it seems to me that only the UK is a real tuff country for CAS transits
You can add Italy & Spain, although for the latter it’s just 3 big airports (elsewhere you may even get cleared to destination along your filed VFR route if you use proper waypoints)
If going around CAS means a couple of extra minutes of flight, I tend to go around by default, although, for example, EPPO Poznan cleared me for a crossing without even being asked. One country worth a special mention is Belgium, which is quite full of CAS but the ATC guides you through seamlessly (at least on an international flight).
As regards the UK, not that I have a lot of VFR flying experience there, but going around CAS near London never seemed unreasonable (assuming I have a GPS onboard). EGVN Brize Norton has a clearly worded transiting guide (not sure whether this one is current) and has never refused a transit to me.
Ibra wrote:
You can add Italy & Spain, although for the latter it’s just 3 big airports (elsewhere you may even get cleared to destination along your filed VFR route if you use proper waypoints)To be honest, I don’t have any experiences with Spain, but Italy is really not so tuff to be honest. Yes, they have some unpleasant class A airspaces and their FIS is quite chaotic, which also likes to tease you with ETA calculation tasks all over again. But on the other hand, Italian ATC will let you pass almost any desired class D airspace, and they even might allow you into class A on high altitudes towards the Alps. They also offer joined-up services, so the next controller is already informed about your requests and/or routing. You just need to refuse those 1.000 ft AGL ‘instructions’ from FIS, as long as you don’t have class A on your flight path. Instead, tell them you want class D crossings or contact the appropriate ATC-unit directly. All in all, Italy is really not so bad at all, even when FIS might drive you crazy.
I imagine the main issue is “ugly patchy airspace” corners with Class A and useless FIS would be a good summary?
Unlike countries with “beautiful smooth airspace” like Germany, Sweden, USA…and good FIS
