That's sort of a 'loaded' question, there are just a lot of 'variables' with it. In a very rough, general sort of way, an electronic device (radio/amplifier/whatever) is designed around the tube or transistor it uses. If the manufacturer of that tube/transistor keep things the same (sort of), then exchanging that tube/transistor isn't that big a deal, some, but not really huge. No two manufacturers do things exactly alike, so there are 'differences' in their products. This one may not 'fit' as well as that one. It's the 'little' things. For instance, a particular brand of tube works fine if it's laying on it's side in a particular amplifier. Another brand 'dies' almost immediately, even though it's the same type of tube. The reason being that the tube's structure inside that glass is turned 90 degrees. When those elements in there get hot and 'sag', they short out. Either can be -made- to work, but it takes some effort to make the changes.
When the Chinese tubes first started showing up they had 'problems', they just weren't as 'close-enough' to the older tubes in general use. Naturally, they got a 'bad-rep' and it's taken a long time for that to sort of 'wash off', come up to what's considered as 'standard'. Same for the Russian tubes. It just takes a while for everyone to get on the same 'page', sort of.
So the Chinese and Russian tubes aren't all junk. You'd certainly better read the 'fine print' though!
- 'Doc