They don't maintain proficiency in basic airmanship, IFR flight and/or multi-engine ops, thereby putting the partnership's assets at undue risk.
They don't share your attitudes on maintenance, upgrades, airworthiness, cleanliness, operation strategy, convenience and/or fairness, any of which may sabotage such a marriage of convenience.
They bail out after 9 months, rendering moot all of the time, money and effort you invested to create a partnership, find the right airplane, buy said airplane, get it up to standards, find it a home, put the corporation together, etc.
They treat the airplane like a rental, with all that implies.
They always seem to want to use it when you want to use it.
The single biggest factor is the partners involved. Good partners can make just about anything work. Anything less and you are headed for an expensive divorce.