if you sued them you must have had problems...
but your missing his entire point. we don't recommend registry cleaners. They NEVER "clean" the registry, they never make the PC faster, and they are completely unnecessary. It doesn't matter wether your aunt and uncle use it, and wether it can cause problems accidentally or on purpose, it's still a registry cleaner.
as you said yourself- it can make mistakes. Why bother running it at all if said mistakes can cause either a non-booting situation, driver issues, or worse, all in the name of what- a few kilobytes of disk space.
the point is not wether it's malicious, or a scam... the point is it's a registry cleaner and they are unnecessary and wholly useless. Especially- as in this case, when they don't take into accoutn x64 file system and registry redirection, causing it to decide that certain files don't exist, and thus declare certain keys invalid.