There's nothing particularly horrible about them, they're just newer parties so they don't have much support, and most people in the US are locked into the idea that only the two big parties are viable.
The Green party has a great stance on environmentalism, but they're kind of a one-issue party. I didn't pay much attention to Jill Stein this time around, but in she's been on the ballot in the past and back then I noticed a significant lack of information on her website about international relations issues. I strongly support the environmentalism initiative, but if you want to be president of the US you need to have SOME opinion on international relations.
The Libertarian party's main issue is "less government". In pretty much all respects. I've seen Libertarian candidates argue for the repeal of things like environmental regulations and civil rights laws that make it illegal for businesses to hire people based on their race. They think responsibility for these issues should be devolved to the state or local level instead of the federal.