it's not better as they fundamentally fail to capture the atmosphere and look of a scene

this means that for some use cases (early QA, design 3D design tweaks before the final graphic is available etc.) they are fully useless

it's both viable and strongly preferable to track placeholders in some consistent way unrelated to their looks (e.g. have a bool property associated with each placeholder). Or else you might overlook some rarely seen corner cases textures when doing the final cleanup

so no, placeholder don't need to be obvious at all, and like mentioned them looking out of place can be an issues for some usages. Having something resembling the final design is better _iff_ it's cheap to do.

so no they aren't failing, they are succeeding, if you have proper tooling and don't rely on a crutch like "I will surely notice them because they look bad"