Are you talking about hosting different clients into different addon domains under a hosting account that you control?
That's probably a bad idea.
Each of your clients should probably have their own hosting account. Now if that specific client wants to have multiple websites then addon domains might be a cost effective solution.
Keep in mind, addon domains all share resources of their parent account and any other addon domains on the parent account. This essentially means that if one addon domain gets hacked or compromised (i.e. an extremely weak WordPress admin password) then all of the other addon domains under the parent account (and the parent account itself) would also become vulnerable.
You cannot completely isolate addon domains from each other.
If you are talking about multiple different clients having their own addon domain, then why should client2 have to pay for client1's incompetence when client1 allows their website to be hacked or compromised?
But if you're talking about the same client, if they let addon1 become hacked or compromised, which leads to addon2 being hacked or compromised, then this is just a natural progression of incompetence.