I said "If they made it a few years, ironed out the kinks and then just they leave it alone, it's less likely to break". I have the same concerns about all new software from any provider. The same concerns applied to Google Checkout and Amazon Payments when they were new.

I just happen to run a startup & have, as expected, observed this fact about software engineering.

However, given the smaller teams and conflicting demands on attention for a startup, the problems may be more pronounced. Furthermore, billing is an industry filled with high degrees of both incidental AND inherent complexity. Navigating that complexity requires institutional knowledge that must be built over time.