Nango is based on Pizzly an existing OSS project which they built on top of. We're building it from the ground up.
Even though they seem to have more integrations, our integration support is better than them in terms of the depth of use-cases allowed (more standard objects supported, custom properties, field mapping support, custom objects (soon) etc).
A few prospects of ours tried out Nango for this use-case and then came to us eventually.
* vs Merge - we'd be able to fly past the number of integrations offered by them being an OSS product especially because of community contributed integrations. Being a developer first product, open-source is the way to build the best product in this category.
Integrations inevitably have edge cases that you would run into and you as a customer might require Merge to behave in a certain way. The typical response at a closed-source SaaS company would be that its on "their roadmap", never to get back again. This holds you tightly with their roadmap velocity and you're locked into a vendor.
Being an open-source product you, the engineer, will be able to fix or add integrations right away in the worst case if nothing else. This way of operating is very powerful we think.
Also, we don't cost you an arm and a leg :)