Joseph:
It’s a made up issue and the people that ever make any hay of it, besides being excruciating few as Sam said, are only mo zionists angry at Satmar’s shitta on zionism. Sam and no one will be able to name any mainstream, semi-meainstream or even semi-semi-mainstream name that has any issue with Shabbos and Satmar. In fact, the issue has nothing to do with Satmar. The kehilos in Hungary at-large as well as many other places in Eastern Europe had zmanim that Shabbos started and ended later than the common American zman. It not only is a 100% legitimate shitta, but it has nothing to do with Satmar or chasidim in general as it was held by Oberlanders and many other non-chasidic kehilos in Europe.
Is the Chofetz Chaim (siman 261, Buir Halacha D”H “Mitchilas HasShkiyah”) mainstream enough for you?
“…mi’yad achar haschulas ha’shkiyah, haynu mi’shuh sheha’chamah niskaseh me’eynainu, ho issur gemur shelo laasos melacha, vi’chas vi’shaom lihukel ba’zeh, di’ho safek issur skilah…”
“… right after the beginning of shkiyah, which is from the time that the sun is hidden from our eyes, there is an absolute prohibition to not do melacha, and heaven forbid to be lenient in this, for it is a possible prohibition punishable by stoning…”