I've setup integration for dozens of payment processors and I would say by far Solid Trust Pay is the worst.
Last I worked on one of their accounts was about a year ago. Everything seemed outdated with poor features (comparing the interface and the API). That was my personal opinion but I set it up anyways as my clients asked for it. Not a single one of the clients that have paid me to setup STP have ever used it. They all switched to another payment processor because of issues STP has regarding setup of accounts, etc.
So, from my perspective as a developer and from my clients perspective as a member of STP they are an all around bad payment processor.