Was the emergence of countries based on a single ethnical/linguistic group a good thing?

No, the emergence of Nationalism only deepened the divisions between neighbouring countries. It also artificially lumped together people who had substantial cultural differences, sometimes by eradicating minority cultures altogether. It is hard to believe today that once Breton was the everyday language of Brittany or Gaelic the majority tongue in western Scotland.

While states were made up of very varied peoples only united by having a common ruler they were much better at dealing with diversity and felt little need to make everyone conform to a single system. Alsace and Lorraine sat happily within the dominions of the French king, and he had no wish to force them to speak French. German might have been the administrative language of the Austrian Empire, but whole regiments used their regional language. Even today India can flourish despite its huge diversity not by making everyone the same but by allowing people to be different. It has even recently split one of its states because the minority felt it was being discriminated against.

Nationalism always gives rise to feelings of superiority to other groups, especially the minorities within the Nation. Their culture is denigrated, they often become the scapegoat for the Nation's ills.

As soon as you call a state by the name of the dominant group you then make the minority outsiders. When the Ottoman Empire became Turkey the implication was that it was not the home of the Armenians, Greeks, Kurds and other minorities who had actually lived there since before the Turks arrived. The consequences were tragic and go on to this day.

Nationalism was what led to WWII as Germany set about uniting all the scattered Germans across Europe (whether they wanted it or not) and treated the Slav peoples as an subject race to be culled to make room for Germans.

Good or bad, it is the natural thing (emergence of countries based on a single ethnical/linguistic group).

The judgemental stance stems from human preferences & bias. My opinion is different from yours. But we both agree that Italians reside in Italy and manage their affairs and not some others, even partly. Ideology colours the issues, gives an opening for outsiders to meddle, indulge in mischief an all that. Even religion does that. On both sides of the international boundary, the Punjabis (speaking the same language & ethnically of same stock) are same but divided by religion, or Bengalis for that matter. But they say it is good.

I beleive it was a natural thing. If we still lived in a Pangea world, things would be very different, but in early history with the drifting of the continents apart, it forced people to come together in groups due to geography. These would then have the same language and ethnicity as time went on.

Yes, it may cause a bigotry, but it is the natural way considering the early history of man.

Depends on who you ask. It has seemed to provide the greatest stability and quality of life. Though it is a very new phenomenon. The word is 'ethnic,' by the way.

No

Both Nazi Germany and Zionist Israel persecute anybody of a different race