I have some monads from when I was less experienced that are screaming to be turned into that. I'm keeping them on my to-do list for now, but it would be a gain. (But I have a logger to write, and will probably adapt to something like it.)
The main downside is complexity. It gets hard to reason about it fast. As types depend on more things, and grow bigger, error messages are hard to read, parenthesis abound, and people really start switching off. Type synonyms help here, but are not a panacea. (A similar problem for your "hard of debug" complaint.)
The COMEFROM style is mainstream on FP, so people are used and expect this kind of thing. That means that your functions better support doing stuff you never told it to do, because it will, you know it, and you'll adapt accordingly. It's normally not a problem, notice the "normally" there.
The main downside is complexity. It gets hard to reason about it fast. As types depend on more things, and grow bigger, error messages are hard to read, parenthesis abound, and people really start switching off. Type synonyms help here, but are not a panacea. (A similar problem for your "hard of debug" complaint.)
The COMEFROM style is mainstream on FP, so people are used and expect this kind of thing. That means that your functions better support doing stuff you never told it to do, because it will, you know it, and you'll adapt accordingly. It's normally not a problem, notice the "normally" there.