To be clear, I'm talking about scraping. I think the sibling commenter is talking about developing competing products via reverse engineering ("clean room implementation"). I am also not a lawyer, so I can only tell you the guidance I received from one for the projects I worked on.

Technically speaking you can scrape data in a legally defensible way if you do not need to accept any terms of service explicitly prohibiting scraping in the course of grabbing the data. The distinction is that browsewrap T&C have plausible deniability, but clickwrap T&C do not. And if you receive a cease and desist order, you abide by it with a mea culpa. This also means you don't scrape so loudly as to be noticed, which has the happy side effect of probably not disrupting the target's service.

But again: The grey areas of ethics are a separate question from legality. Please engage a lawyer for your specific work.

Depends on jurisdiction OP operates in, but clean room reverse engineering is one technique, i.e. OP reverse engineers a piece of software, documents how it works at a high level, and hands off the documentation to legal counsel who reviews it for IP infringement etc. The doc is then handed off to a completely different team/engineer who implements the API contract documented by OP.