It appears that Tray.io is mostly situated for enterprises that wish to automate their API connectors using a GUI tool they created; allowing easy creation of APIs for businesses.

But compared to the competition however, Tray.io is has less supported APIs, no starter / free trials or signups (You must get a quote from them) and they are somewhat less flexible in customization options when compared with the other alternatives.

At this point, I would choose Zapier over Tray.io, since Zapier is basically the same thing but can be used by many developers from freelance to enterprise, free trial to try it out and supports more connectors. I cannot comment on Parabola.io though as I have not used it yet.