(2) Partnerships can be formed without any documentation. While there are some tax issues, sales tax specifically, those issues are for a different post. Instead, a partnership can be formed and have direction as to how the business should proceed. Without a partnership agreement, there is an open issue of what happens if the best or the worst should happen. Without a partnership agreement, the answer will probably be litigation; with a partnership agreement the answer may be litigation but there is also the agreement to rely upon.
(3) There is a lack of clear authority for the partnership. A partnership, formed without any of the formalities on purpose or on accident, does not have any hierarchy. In an LLC, there are the members and perhaps the manager(s). A corporation has its board of governors and the C-suite. A partnership has equal partners. Each is equally able to agree to a legally binding contract for the partnership. To hire, to fire, to perform all the essential tasks of running a business. This could easily create problems where each partner agrees separately to two different obligations for different prices and the partnership will be obligated to perform both.
A partnership may be the best way to form a corporation for you. However, there are a number of concerns associated with the ease in which a partnership is created. If a partnership is the course you take, spend the money on a local attorney and get a partnership agreement. It will save some headaches down the line.