Someone who will say anything to avoid rocking the boat. They will go against their own morals and standards to make statements to please others. They shouldn't be trusted and confiding in them isn't a good idea. They are liked by many due to the fact they always have good things to say, but are not trusted by the smart ones who have figured them out. Fence riders are phony butt kissers. We all know a few, most are politicians.
Wow!..You look great in that dress!
OMG!..Doesn't she look awful in that dress!
It's not a matter of them not being able to make up their mind, fence riders say what's needed/wanted to be heard.