It's an organization that has repeatedly been sued for defamation, and paid out tens of millions of dollars for doing do. Among some of the most egregious episodes were when they recruited a family to wiretap their neighbors. To many, they embody the tendency of using allegations of hate and bigotry to justify authoritarian measures.
Sure, they might have a bit of a blind spot that makes them overly favorable toward Israel, but in a sea of bad actors, they're one of the few groups acting in good faith.
The ADL spied extensively on not only pro-Palestinian activists, but also on anti-South African apartheid activists, presumably because they didn't want their critiques being used on Israel.
I was curious, so I read the articles. For something intended to critique the ADL, they spend a lot of time listing malign actions of the government of Israel. Which is annoying, because... I don't care. I oppose Israel having cooperated with South Africa in the 60s and 70s, but that has pretty much nothing to do with the ADL in 2019.
The spying seems like a more damning charge. But the word is doing a lot of work: most prominent political organizations maintain rolodexes of reporters and activists, along with their stances and perspectives. It's part of what makes them effective--you can rest assured that e.g. CAIR does the same. Characterizing it as spying is a bit overwrought.
We're talking about the ADL being a Zionist organization, and the viewpoints that caused individuals to be the subjects of spying (or whatever you want to call it) clearly indicates a pro-Israel and anti-Palestinian bias. I don't think anyone would argue CAIR isn't a pro-Islamic organization with accompanying biases.
Why is that bad?