Just generally the sandbox was horrible, it had a weird 'double login' system where it often got confused between whether you were 'inside' the sandbox logged in as a fake user or outside logged in as your paypal account.
And you're right about their woefully confusing documentation, it was definitely hard to understand which payment flow you were trying to use and which was appropriate.
Maybe it's a bit different now, but I haven't looked back. :-D
PayPal tried to "modernize" their old crufty SOAP API with a REST-based API but ended up creating a separate half-functional, poorly documented, buggy layer on top of it that actually loses payments sometimes. I processed millions of payments through it and actually have their support people admitting the problems.
Their only hope is a reverse-takeover from Braintree. I'm not holding my breath; I've done my last PayPal integration. Forget all the "paypal will hold your money" concerns - the real problem is that they will loose your money in the wonky Rube Goldberg machine they've created to process payments.
The project just ended up getting scrapped.