Co owners can also axe certain items even if they didnt place them. This includes but is not limited to : Guildstones, Gozas, simple deco deeds (forges, ovens) and probably a bit more that im forgetting. With that, there has literally only been one person in my entire UO career that I have trusted enough to Co-Own. #trustissues.
A co owner could absolutely ruin a house if they flipped a switch...
I like the rule: if a friend/co owner didnt place/lockdown/secure an item he cant remove, change security, or unlock said item. I wish it applied to all items the same.