What are your challenges when building integrations?

I am looking into building a tool that will simplify API based integration building.

I built integrations in the past, and for me, personally, the biggest pain points were:

• the OAUTH2 flow that is accessible only from a browser
• the token management can become a pain, especially when you have to deal with the refresh tokens
• you have to move a lot data around between separate services to be able to access the underlying APIs, which makes background processing for these APIs more difficult that they should be
• each API has it's own specification and structure, especially around permissions
• I never bothered with building the whole permission upgrade/downgrade flows because I find it too complicated.

What else?