Bad! Good! Bad! Good!
Okay, let's see if we can get this straight.
1. Separating Scotland from England is bad.
2. Separating Czechia from Slovakia was good.
3. Rejoining Crimea to Russia is bad.
4. Rejoining East Germany to West Germany was good.
All of these are stated as absolute rules, with the implication that every other separation or rejoining should also be bad or good. Immediately after implying that all separations are good, the same Expert implies that all separations are bad.
The contradiction between 3 and 4 is especially annoying because the situations are closely parallel, and because the loudest complainers about Russia repossessing Crimea are East Germans, who benefitted massively from West repossessing East.
Experts, of course, are not thinking about the welfare of the people in those countries. Experts are only trying to maximize their own political power and wealth.
Leaving the experts aside, is there a usable absolute rule? I doubt it.
But there are several STRONG general rules, with some exceptions and some limits for each. GENERALLY: Smaller units are better than larger units. Mono-ethnic countries are better than multi-ethnic countries. An entity that has been independent through most of history should be independent. An entity that has been part of a bigger country through most of history should remain in that position.