Quick feedback for you guys...
I dared updating to 3.4 ... fingers crossed... and ... nothing to declare
I also googled "joomla 3. 4 update issues/problems", only one problem mentioned was in the French joomla forums concerning a language filter problem and URL's ending with a double // (
http://www.yoursite.com//fr ).
Fix mentioned is: go to language manager -> content -> open each language -> save and close + also check language filter
I have 11 languages on my website and the update did not create any problem (not even language)
Just waiting now for the updates of the installed languages to current joomla version (some are already available).
Best regards