What is it? Genuinely curious, as I am using and relying on LetsEncrypt

The cost to build and maintain the infrastructure to handle renewal and tests to verify such.

I would say the technical hurdle is about equal to comodo. What other cost is there?

Comodo is a renewal and updating an artifact (i.e. the actual cert), then you're done, vs. having to handle LE's API to deal with doing it 'automatically' and then carrying that forward. My point is simply that the cost is not zero.

You can still just replace the cert manually like you would any other CA. It's just there's no official GUI to do it with LE and you have to do it more often.

There's already mature, widely adopted tools like certbot and acme.sh that do this automatically for you.

It may not be zero, but it's no more than Comodo.