I've been playing a GC '44 and experienced Rumanian surrender in all its disastrous implications... In general, I think the rules for it are fairly decent--- the actual historical surrender came very quickly just as it does in the game--- but there are a few problems. The first is that all rails in Rumania on hexes that flip convert automatically to fully repaired Soviet rail. I imagine they should instead be damaged. Second, as happened to me (and I only blame myself for not studying the surrender rules more), the flipping of Rumania can cause the towns in NW Rumania that start a Hungarian surrender check to switch sides, and Hungary can surrender immediately on the same turn as Rumania. Actually, I'm not entirely sure how the Hungarian surrender happened, as I had an SS and normal division in Budapest which would seem to guarantee me passing every surrender check, but I'm probably missing something... I do think that probably the Hungarian surrender locations need to default to Axis control when Rumania surrenders to prevent this kind of chain reaction, or at the very least there needs to be a time delay between Rumanian and Hungarian surrender.
That said, now having experienced the surrender process once, I've come to the conclusion that Germany can't risk fighting in Rumania until AFTER the Rumanians surrender, as the risk is just too great that something disastrous will happen, nor can one ever have enough units in Bucharest to negate the surrender check. But the surrender can be properly managed--- largely one would need to create a ZOC screen of German units from the Hungarian border south along the rail line through Budapest and to the Bulgarian border, as well as make sure all Rumanian units are adjacent to German ones so that they don't switch sides. I think with that strategy, one can then safely withdraw from the exposed parts of Rumania, keep Hungary in the war, and utilize the rough terrain in Transylvania for a fairly decent southern anchor to one's front. Of course, I'm not sure if it really makes sense to have a disastrous event like the Rumanian surrender be able to be "managed" in game terms, but so it goes...