There's nothing objectively wrong with the hashtag itself.
It's mainly shitty at this point because it's been mainly used by shitty people who have associated it with shitty behavior to the point people reflexively roll their eyes at the slogan. It's the "Occupy" of race relations at this point.
I think that is inherent to the matter.
Every new movement attracts nut-jobs, this is pretty much a law. People with extreme ideologies flock to every new popular movement, that they can abuse as a mouthpiece for their ideas. Start you own club that wants to make your neighborhood prettier and you get at least one of those "Greater Good" guys from Hot Fuzz. Take social media into the mix and this gets one hundred times worse, because now you got the power to instantly reach every antisocial idiot with a smartphone. (And these days you can't go without social media.)
And of course those extremists are the people who 'shout' the loudest. For every ten people tweeting or posting something you would expect from the 'normal range' of the mental and political spectrum, you got at least one idiot demanding to #KillAllX - tweeting it ten times a day. Also, these are the messages people tend to notice.
I don't think these movements work for normal people. Before you even know it, a cause can have thousands of people rallying around it and everyone has a different opinion about what this 'cause' is. You need to have a list of demands before you start a movement, so it is more concrete. But most of these things start out as #IDonTLikeThis, it catches on because other people also don't like it and when it has caught on, there are already a million of people out there claiming that hashtag for themselves. So it is usually more like '#SomethingSomethingRacism' or '#SomethingSomethingFemism' ... '#SomethingSomethingSomethingism'.
Of course, because you movement isn't very well defined it also becomes open to attacks from 'the other side'.
#MakeMyNeighborhoodNiceAgain? Too bad! You are a obviously garden gnome nazi, who wants to tell people how their front lawn has to look like.
Then comes the point where 'normal' people are so alienated with the entire thing, they just step away from it, leaving the stage to the nutjobs.
Look at #GamerGate for instance. Ask someone if he would like to have ethics in game journalism, the answer will be 'Yes'.
Ask that person if he is Pro-GG and the answer will be 'Uhm... well... aw... no.'