The main difference is that Merge pre-builds all integrations, with limited customization capabilities, while Nango gives you full control over the code of the integrations.
Not trying to be snarky, but I'm not sure why this matters? Isn't the main value prop of a service like nango or Merge just the unified API and features around it (e.g. unified webhooks for events etc)? Can you help me understand why this is important?
Standard pre-built integrations are ok to get started. But rapidly customers ask for more integration capabilities, which are not supported by standard integrations. At this point, SaaS companies need to build custom integrations that fit exactly what they customers want.