Despite the "ethnic cleansing" that occured in the process of the fall of the Austro-Hungarian Empire and the formation of the modern nation-states, you still see German surnames in countries like the Czech Republic, Hungary or Romania. Unlike other "legacy" minorities (Hungarians in Romania, Slovaks in Hungary) they don't seem to be considered a foreign ethnic minority or be at the center of any ethnic tensions. Are they descendants of mixed people who were already assimilated to the local language/ethnicity or the descendants of ethnic Germans who chose to stay and assimilate?
I can only talk about the Germans in Romania.
First of all, there are three main differences between the Hungarians and Germans here from a Romanian perspective.
First of all, the historical relation of Romanians with these two ethnicities since 1848 was different. We quarreled with Hungarians since then as any two neighboring ethnicities did. As the Hungarians wanted to magyarize us and we had a process of Romanization after 1918. We did not had a border with Germany or Austria and thus the Germans were not seen as a danger to Romanian territorial integrity or territorial aspirations. Furthermore, Germans were, to some extent, also subject to the process of magyarization until 1918 and thus we were in the same boat as them. Even the disappointment of unfulfilled promises in 1918 toward the German minority were not seen as "dangerous" because we did not had a common border with their kin state.
Secondly, there were always more Hungarians than Germans in Romania (in 1930 we had 7.89% Hungarian and only 4.13% Germans). Being the biggest minority in the age of nationalism, was a downside.
Thirdly there is a big difference in the image that the two minorities have today. Germans are seen as hardworking and correct par excellence (those traits are mostly generic when talking about Germans in most countries) while there are not positive images of Hungarians. Even those who are not nationalists and do not see Hungarians in a negative way, can not mention a positive trait that is specific to them. They are seen as average in everything. Maybe sometimes you say that have a great cuisine, but that is it. Also, Germans are seen by those who are very pro-western as a link to having ties with the west, while Hungarians are not seen as a link to the west because they are in our neighborhood.
But why those positive stereotypes for Germans and not for Hungarians? Besides the historically strained relations with Hungarians, their exodus since the 70's played a part. Right now we do not have that many Germans. In 2011 we had 6.10% Hungarians and 0.17% Germans. By being such a tiny minority, the possible ethnic conflicts/misunderstandings are low.
As for the assimilationist stuff at the end. Germans until the 80's had a very closed community. I was involved in a project from IKGS a few years ago and we conducted interviews with both German and Romanian older people and asked things about their daily life and ethnic relations in mixed villages. Romanians, Germans and Roma people lived in many cases in separated parts of the village and did not really interacted that much with the other group. Mixed marriages was rare and only became more prevalent since the 80's.