There are many other services that provide similar things, such as Stripe. However, when I evaluate these marketplace payment providers, I look at who their current users are and in this case, WePay has provided services to many other high-volume payment customers.
PayPal has it's own niche, and have more stand alone options, but they are confusing to navigate unless you are in their system a lot. Integration can take a lot more time to do and test out. Plus WePay charges less than PayPal does which in this case is an advantage.
WePay is friendlier to the customer than Stripe or PayPal - so much easier to sign up for and use. Standard gateways are more flexible - they provide many more options on pricing, and the ability to choose the underlying processor - which is great, but also significantly more complicated for the customer.
WePay directly integrates with Classy and that was our sole reason for switching from iATS. iATS has several features but is also fairly complex. iATS has a great customer support team to help though. If we were not required to switch payment processors by Classy, I don't know if we would have selected WePay or not. It does the job, but does not appear to be anything special one way or the other.
WePay is a payment processing and mobile point-of-sale (mpos) software solution with features such as facilitating payments between buyers & sellers and instantly enabling users to accept donations. WePay was acquired by Chase in 2017.