The positions may not be, but voting for anything but R or D is very controversial, because by voting third party in the US, you are helping the choice you like least win.
There is exactly zero chance that a third party will gain any meaningful amount of power, so all you are doing is throwing your vote away.
Maybe you're one of the five people in the country who don't actually care which of those two will win, but for some crazy reason still go to the polls - but if you're not one of those five, the best you can hope out of a third party is to produce better candidates in your preferred party's primary races.