I have in the past criticized, and probably shall criticize in the future, activists. (Click here for a recent example.) But I should point out they have a hard task, they usually believe in what they're advocating for, and even if one disagrees with them, they are usually raising a good point or confronting an issue that needs to be confronted.
"Usually" isn't always. There are some causes that are so morally reprehensible that the activist deserves to be ignored or even counter-demonstrated against. Or if the causes aren't necessarily morally reprehensible, sometimes the means advocated are. I do suggest, though, that just because we disagree strongly with someone doesn't make them morally reprehensible.
Activists' may not have super pure motives. Some really are on a power trip and care more about being an activist about their cause. Some are being paid and some of those (though probably many fewer) are being paid handily, either in money or in prestige or power or ego stroking. Some may come across as unsufferably self-righteous. (Pro-tip to activists: when you say, "I could be making a lot more money doing something else," you should at the very least expect to receive some eye rolls, even if what you say is true. One, it's probably less true than you think it is. Two, even if it is true, the people you are talking to may not have the same options and may find your nobility condescending.)
But none of us has pure motives. Most of us, almost all of us, do the right thing at least occasionally. I submit that none of us does so out of purely altruistic or self-sacrificing motivations. Even if I'm wrong about "none of us," I'm on more solid ground to say "most of us" or "almost all of us." At any rate, we (I) should think twice before judging.
My own brief forays into activism were exhausting. I once participated in a boycott campaign against a bank I believed was underwriting unfair labor practices. I once canvassed voters. I attended a couple rallies against the Iraq war. And I participated in an ill-advised strike my colleagues called a few years ago. None of these events was fun. I did meet some people who actually seemed to enjoy them. But for the most part, those actions were drudgery for me, and I found them in some ways to be morally compromising. However, with the exception of the strike at my workplace, I believed in the cause at the time. (I have a different view on most of those issues now, or at least on whether I should have done what I did.)
It's easy to criticize activists. We should criticize activists. They're voluntarily engaging in an activity that puts them out there. But we should remember what they're doing is hard.