I hear you. Those are my exact concerns with a partnership as well. If I do pull the trigger on this partnership I am going to take the advice of my accountant and make sure we layout, in writing, what happens if the partnership goes bad. Better to do it while we are on good terms than trying to sort it out should things go bad.
As for the VC side of things, I have looked at a few start-up companies as well (more as an Angel Investor than a VC) I tend to want to limit my exposure on any one deal to about $250K. In addition to the problems you have cited I also find that coming to terms on a baseline valuation for investment purposes with a ground-level start up is always a tricky situation.... Most people want put their valuation way above where it seems reasonable to me.