Based on many years of experience, using a large number different registry cleaners on tens of computers, I have found the following. I would say that at least 99.9% of the time, cleaning the registry will not make a computer run faster or fix any problems you are experiencing. No matter how many hundreds or even thousands of so called errors, that a registry cleaner finds, it is exceptionally rare for them to be causing any problems or slowing down your computer. As a result, the vast majority of the time, cleaning the registry is a waste of time. No only that, but just about every registry cleaner ever written (except for a few exceptions) has issues with false positives. This means that sometimes they will think that valid registry keys are errors and will want to delete them. Deleting such keys can sometimes cause problems. Even registry cleaners like the one in CCleaner which is widely regarded as being safe, has issues with false positives.
There are occasions when cleaning the registry can fix problems or make your computer run faster, but there are very rare. An example of this is the following. When I'm doing a cleanup on a customer's computer which is running slower than it should, I will clean the registry as a last resort, if after doing my best to diagnose and find the cause of the slowdowns, it is still running slowly. Every time I've done this, the computer has run just as slowly afterwards.
If you use a safe registry cleaner (one which has no issues with false positives), you won't cause any problems. But, on the other hand, most likely you won't fix any problems either.
I used Ashampoo years ago too. I don't like it so much these days. I avoid solely using a programs own uninstaller, as I want to remove any leftovers which could potentially cause issues, as I plan on not doing a clean install of Windows for many years.
However, unless you are regularly installing and uninstalling software, then just using a programs own uninstaller will be fine. The one exception, is security software. After uninstalling an antivirus, download and run the vendor's own removal tool, to ensure that all leftovers are removed. This is very important, as sometimes there will be leftover drivers which can cause issues with other antiviruses, such as slowdowns or even blue screens of death.