What are "grime files" ? Do you mean the temp folder? What are 'leftovers" ?
What does "clean the registry" mean? Does it delete entries judged to be superfluous,
or does it just compress the registry and save some space.
I'm glad that you say it behaves in a non-aggressive manner, but I would ask you why
are there so many users having problems?
If, after this cleaning, the PC boots up and something is now not working, then, avast cleanup did too much.
This is the problem with all these kinds of program (cleaners). I think it really takes a tech user to use them properly -
somebody to look at the proposed actions and be a final judge of running them or not. And even then, there
will still be an occasional mistake.
Do you really think it's a good idea to let this program loose on the general (mostly novice) avast community?
Is it a good idea to use it in Windows 10? It's early days for the OS, it has enough of it's own problems
with things not fully stable. Running avast cleanup in here can only complicate the situation, surely?