I think it is more that the documentation is poor, and that they have multiple versions of things so Googling can bring back answers that don't apply. PayPal also lacks many features that are easy in Stripe like changing billing plans or prices. Of course the REST API is just a REST API, so calling it isn't any harder than calling Stripe's REST API. We did a SaaS subscription implementation in Stripe and PayPal, and while it ultimately took less code written to implement PayPal, it took at maybe 5 times as long to complete. The time wasn't the issue, it was that it was insanely unpleasant. It made me hate PayPal - I flip off their offices when I drive by.