Shared hosting can be great economically, particularly a provider that allows hosting of unlimited domains. I used to think one of the beauties of shared hosting was also that you could have a bizillion sub-domains and WordPress instances for free. Then I learned that you also need to keep them all up to date.
It’s called cross-contamination and Sucrui has a good article about it.
I recommend the Sucuri scanner. It identified a very subtle problem that was plaguing my site. Fortunately I found the problem before Google had blacklisted me or the embedded content had polluted the search results associated with my site. That is a quick way to get blacklisted by Google.