For instance, Micro provides a persistent form of key-value storage. If my product focuses on a very specific audience (say medicine), but also exposes a way for doctors to store arbitrary key-value pairs, could that be argued in court to be competition?
The definition of competition in the Polyform Shield license is never explicitly stated, and in fact, it specifically calls out that "Applications can compete with services".
Whereas the MIT and Apache license are pretty explicitly clear in what the terms of use are, Polyform seems to be fairly vague there, which is potentially something you wouldn't want for a legal license.
> Any purpose is a permitted purpose, except for providing any product that competes with the software or any product the licensor or any of its affiliates provides using the software.
You're not just unable to compete with Polyform, you're unable to compete with Polyform's affiliates.