I know the Bessarabian Germans are generally considered Russian-Germans because they settled the territory after it became part of the Russian Empire in 1812. Also like the Volga Germans (22 July 1763) and Black Sea Germans (20 February 1804) they received a guarantee of privileges and rights from the Russian government on 29 November 1813. But, I view the Bessarabian Germans in many ways as more similar to the Baltic Germans than the Volga Germans and Black Sea Germans. In particular the modern history of the Bessarabian Germans is much closer to that of the ethnic Germans from Estonia and Latvia than it is to those from the Volga and Black Sea regions. Like the Baltic Germans and unlike the ethnic Germans living in the Volga region, Ukraine, the North Caucasus, Crimea, Siberia and other parts of the Russian Empire, the Bessarabian Germans never came under Soviet rule for any appreciable length of time.
Instead the Bessarabian Germans came under Romanian rule in January 1918 and thus avoided the horrors of collectivization, dekulakization, famine, and mass executions that afflicted the rest of the Russian-German population in the 1930s. In 1940 after the Soviet annexation of Bessarabia the German government evacuated almost all of the 95,000 ethnic Germans from the territory to areas under German control. The German government also evacuated the ethnic Germans from Eastern Poland, Lithuania, Estonia and Latvia during this time. The Bessarabian Germans thus never experienced the mass deportations, special settlement regime and conscription into the labor army that defined the creation of a modern Russian-German identity. The historical experience of the Bessarabian Germans during most of the 20th century thus differs considerably from the ethnic Germans living in the Soviet Union. The history of the Bessarabian Germans since 1940 is instead much closer to the Volksdeutsche communities evacuated from Lithuania, Latvia, Estonia and Eastern Poland at this time.
Given the huge divergence between the Bessarabian and other Russian-Germans in 1917, I think it might be more useful historically to think of them as a separate group after this time. Their experience from 1917 to 1940 has more in common with Germans living in Transylvania than those living in Ukraine. After 1940 they come under German rule and in the post-war period they resemble the expellees and refugees from Poland and Czechoslovakia far more than they do the Russian-Germans living in Kazakhstan and Siberia. So I would propose that the term Russian-German really should only apply to the Bessarabian Germans for the period of 1813 to 1917. After that they become a separate group from the Russian-Germans whose modern history and identity revolves around persecution in the USSR under Stalin.
Sources for further reading:
Polian, Pavel, Against Their Will: The History and Geography of Forced Migrations in The USSR (Budapest: Central European University Press, 2004).
Prauser, Steffen and Rees, Arfon, eds., The Expulsion of the ‘German’ Communities from Eastern Europe at the End of the Second World War (Florence, Italy: European University Institute, 2004).
Schmidt, Ute, "Germans in Bessarabia: Historical Background and Present-day Relations," South-East Europe Review, March 2008, pp. 307-317.
Stricker, Gerd, "Ethnic Germans in Russia and the Former Soviet Union, " in Wolff, Stefan, ed., German Minorities in Europe: Identity and Cultural Belonging (NY: Berghahn Books, 2000).