The problem is that to get an SMSC that is respected by all of the operators requires a negotiation with an aggregator who has done contracts with all of the operators. The 2nd order problem is that you don't know the quality of the relationship an aggregator has with operators until you're a customer. All carriers have a prioritization in their SMSC's and, necessarily, no aggregator is in position 1 on all carriers.

The result is fairly untraceable failures.

Source: former telecom PM.