YU thoroughly removes installed applications via the applications uninstaller and THEN scans the registry and file system for known changes based on the type of installer/uninstaller used. Works really well.
It depends on your level of demand.
If you are curious about what has been added to your computer or you want an absolute clean uninstallation later, using uninstaller like Ashampoo Uninstaller or Total Uninstall is always a better choice.
Ashampoo I use very rarely
Me too! It is too slow.
Instead I always use Total Uninstall which is much faster and can work in both ways that you have described.
I NEVER use it to actually perform an uninstall, though... Doing so is asking for trouble because any background process that is running while creating the uninstallation snapshot that makes changes to the registry during that time is going to be corrupted...
I think your 'believe' above is very misleading if not totally wrong.
1) Generally speaking, if there are background programs that need to write crucial info to registry while you are doing program installation - that implies you are not installing under recommanded mode... didn't you be informed/advised to turn OFF unneccesary applications prior to installation?
2) Even if there is exception, say you are unable to turn off some background programs/services that write to file or registry, those writting, normally are non-crucial records of temporary info. Throughout my life of installing so many applications (with proper monitoring of the process) I have never come across a case where I can't easily stop a software (prior to installation) that really write crucial' info at the background.
3) Any uninstaller like Ashampoo (or Total Uninstall) will normally offer feature to exclude background disturbances. I mean what you need to do is to simply create a snapshot of your system while you are not installing anything and do something you normally will while installing application, say open notepad, run window explorer and then just create another snapshot to find out those background I/O either to registry or files, the process is rather simple, what you need to do is just add those found items to the uninstaller's exclude list.
IMO, if one doesn't find uninstaller like Ashampoo Uninstaller or Total Uninstall good (or find them very troublesome to use), he/she generally has very low expectation on how much he/she can monitor a program installation, i.e. I think Windows's ordinary install and uninstall procedure from the control panel is good enough for him/her.