I heard it wasn't so much that the engines were unreliable but the fact that two engines were tied into each propshaft, whereas the 277 went for a more conventional four engined layout.
The DB 606 "power pack" engines caught fire more than the B-29's, which is an anti-accomplishment. However, the very same engines were quite reliable on the He 119 and Me 261; the problem was that the Greif's nacelles were too tight, which forced substantial redesign of key auxiliary systems into badly designed and inadequate forms.
Specifically:
1. In order to save weight and space, the inboard cylinder banks of the two side-by-side engines shared a single exhaust manifold, but the central manifold was not appropriately scaled up to serve twice as many cylinders. This meant it regularly operated at an excessively high temperature, which tended to ignite the normal oil and grease buildup in the engine cowling.
2. The above issue was exacerbated by the tendency of the mechanical fuel injection pumps to reduce their output too slowly when the engines were throttled back, delivering a surplus of fuel which often leaked through faulty pump connection seals. The leaking fuel naturally collected in the bottom of the engine cowlings, where it could also be ignited by the perpetually overheated central exhaust manifold.
3. To save weight and space, there was no firewall, further increasing overheating tendencies and guaranteeing that any engine fire would be uncontained.
4. The oil pump was also badly designed, causing the oil to foam up at high altitude. This compromised engine lubrication, which repeatedly caused the connecting rod bearings to degrade; if this went on long enough, the rods would punch through the crankcase and puncture the oil tanks, causing a full-scale oil spill onto, you guessed it, the constantly-overheated central exhaust manifold.
5. The closely packed installation left insufficient space for fuel, oil, and electrical lines to run, leading to all of the above being constantly faulty. It also meant that maintenance access was poor.