There are not very many negative outcomes for democracies. There tend to be negative outcomes for the bigger parties because big parties are the beneficiaries of FPTP, as they get overrepresented. That of course was the reason that the big parties in New Zealand were opposed to a change to proportional representation.
One fact that I think has already come up in previous questions and comments is that cabinets or governments may not last as long, may not be as stable under proportional representation as under FPTP, and that it also takes longer often to form a cabinet because negotiations have to take place. These are usually connected with the thought that these facts in proportional representation are bad for policy-making, but in fact when you look generally—again as I've said—between majoritarian systems and consensus proportional systems, the consensus proportional systems do just as well or in fact usually better than the majoritarian FPTP systems.