- Hidden prototypes don't show up in upgrade tool "from" options, but still shows up in "to" options - Upgrade tool can't be used on entity with a hidden prototype - It's possible to not use hidden entities, using "hide-built-in" tag, but it makes upgrade tool clutter much worse - Fixing the upgrade tool by removing "placeable_by" for rigor entities requires huge runtime scripting hacks - Pipette breaks - Blueprints break (no cut and paste with external wire connections) - No ghost on entity death As an intermediate step, the hidden/upgradable tradeoff can be made configurable. This likely requires a modding interface request to fix.