Apparently the only negative effects are: you look awful, and the amount of phantoms you can summon are reduced and *apparently* it will be harder for you to see summon signs.
I don't think those are true at all. I've been running around with 99 Hollowing through NG and NG+ and I've always been able to summon 2 phantoms max without using the thing that lets you summon more (and be invaded more). There is a boss where I could only summon one person and other signs would disappear, but that's not because of Hollowing. Also pretty sure the summon sign thing was wrong, because the floors before bosses were always littered with them until I got to a pretty high level.
I'm fairly certain that the only negative effects are purely cosmetic and that the Hollowing stat is mostly for the Hollow Infusion.
http://darksouls3.wiki.fextralife.com/Hollowing