Hello everyone,
a very general question: YNH is based on Debian. Technically, I can still do everything with Debian that would also work without YNH. For example, installing software (apt install …). But how ‘dangerous’ is that?
If I install my personal CLI editor now, this should probably not be a problem. But installing Apache will certainly ruin everything. Where is the limit? Is there any ‘characteristic’ that can be used to judge whether an action is not good for YNH and the entire system?
I know this question is super general and certainly difficult to answer in general terms. But maybe one of you has an idea … Maybe you will also say: don’t do it. 
Then it just confirms my previous caution.
Thank you for your suggestions.