I just have the impression that F1 engines (especially pre-2005) are inherently unreliable, and maintaining high rpms (to keep it competitive) on an oval for any extended period of time would cause it to go bang Before 2005, there were heaps of engine-related retirements every race, and this was under less severe conditions than an oval. Indy engines on the other hand don't rev quite so high, and are more robust.