Momence is a massive product with tons of features, putting it roughly on the same level with MindBody. But this also means that Momence is complex and also pretty expensive. Momence can do a lot... but most of the features it comes with are ones you probably don’t need. And their pricing isn't very straightforward.
Ubindi provides "just the essentials", and it's amazingly user friendly. You won't find "multi-admin accounts" or things like payroll calculators for staff members... but Ubindi will solidly handle class bookings, payments, passes, memberships, attendance, on-demand videos, mass emailing, managing clients... all while still being very easy to use and intuitive. Ubindi is designed for non-techie people who don’t want complicated dashboards, configurations, settings and reports.
Summary: on "features", Momence beats Ubindi. On simplicity and cost, Ubindi beats Momence hands down.