The biggest problem is the lack of legal-to-start, legal-to-finish.
So many opportunities for crews to get stranded at an outstation.
The lack of legal to start legal to finish is going to kill the US carriers in competition with international airlines. Think about it, a BA jet and a Delta jet both take off from Europe on the way to JFK. Both divert to BDL because of bad weather at JFK. When the Delta crew hits the ground in BDL they are dead in the water, but the BA crew can refuel and continue. How long before passengers start to book away fom US carriers?