Boards and categories
One rule decides which you want: a board is something you would publish a separate roadmap for.
This is the only structural decision nolby asks you to make, and there is one sentence that settles it.
A board is something you would publish a separate roadmap and changelog for. A category sorts feedback within one.
What that looks like
One product, sorted by type. One board. Add categories for Bugs, Features, Integrations — whatever your customers actually send you. This is most people, and it is why a single board is the default rather than a limitation.
Several products. A board each. They get their own roadmap, their own changelog and their own embed snippet, so a customer of one is never made to read another's releases. Reuse the same categories inside all of them — categories are defined once for your whole account.
One large product with distinct areas. Either, and the rule decides it: if each area needs its own public roadmap, make them boards. If they share one, make them categories.
Clients, or a private beta. Boards, because only a board can be restricted to people you invite.
Why not just use boards for everything
Because a visitor has to choose a door before they can speak. Four boards for one product means four pages, each of them thin, and someone with a bug report guessing which one you meant. Categories keep the conversation in one place and still let you filter it.
What a category cannot do
Have its own URL, its own roadmap, its own access rules or its own form fields across a whole board. If you need any of those, you wanted a board.