>The problem with this being that group borders are petty flexible and easily manipulated.

Not that flexible -- it takes long, historical processes to change them.

People were living within the same "unified" culture, state mandated et al, forced to be together in USSR and Yugoslavia for 50 to 80 years -- and yet those states exploded into multiple national countries as soon as they had the chance.

Some borders are organic (based on culture, shared history, geographical boundaries etc) and some are not (based on colonial rules drawing lines in a map which include groups with different cultures which have historically been enemies in the same "country": the British have done this time and again, and is also the most common cause of tension in Africa etc).

The USSR was a re-implementation of the Tsarist Russian empire by another name. The peoples yoked together in the USSR had been absorbed by Russian expansion in the 18th & 19th centuries. Yugoslavia was a variation on the same theme. The north had been part of the Austro Hungarian Empire, and the south part of the Ottoman Empire.

That's what I write. Merely getting people to live together under one "union" doesn't mean it will stick, or that they like it, or that they'll identify with it over their prior and existing identities.

Nor (as happened in both cases) that they wont split on the first chance they get.

It's a balance thing, you cannot survive without your environment, but you have to find a suitable one to avoid dying too.