Is hosting multiple domains on one account bad for SEO?

Hey web pros!

I noticed that many hosting providers offer unlimited domains with hosting packages.

If I host a bunch of sites this way, is there any downside? I'm thinking it may affect SEO.

It's usually one main domain and all the others are "add-ons" which are set-up like subdomains.

Ex. PrimarySite.com
addon1.com is also addon1.PrimarySite.com
addon2.com is also addon2.PrimarySite.com

etc..

So any thoughts or knowledge on this?

Thanks