That might always be a problem.
https://support.google.com/nikcollec.../3000951?hl=en
Look also at the chapter "uninstalling the original nik plugins".
If I do it this way, I also remove the directories if the uninstaller didn't. And run CCleaner to check/clean the registry.
And if you're curious, you can do a search in the registry and look for NIK COLLECTION. Don't make changes in the registry.
George