We implemented one of the APIs (can't remember the name, it was something stupid at least) recently, and half way through our implementation the parameters we received back changed name for no apparent reason, and the PayPal sandbox was entirely changed; this resulted in our account becoming broken, constant errors where code was previously working, and it was never resolved.

WorldPay is incredibly outdated, underfeatured, and rarely improved. Their support is pretty decent, however.

With Stripe and Paymill now available, I don't see any reason why people should continue to use what we've been struggling by on for the past few years. They're so much better that I can't even really draw a comparison.