The most obvious risk is misinformation -- e.g. someone claiming to be from Party A, handing out Party A branded leaflets that actually direct people to vote for Party B -- but in practice that doesn't seem to be a significant problem. I'm sure it happens at small scales online, but in person it's hard to get away with, so at least the information seen immediately before voting is accurate. (Though admittedly I have heard of one or two borderline cases, e.g. cards with colour themes chosen to misleadingly hint that they're associated with a particular party.) What is prevalent is 'preference trading' between political parties, as a significant number of voters do seem to more or less blindly follow their party's suggested ordering. This is arguably a problem, but it doesn't usually get too crazy, as preference deals between ideological enemies both look bad and can easily backfire.