The ADL is a Zionist organization. It's hard to take their opinions on ethnonationalism seriously at all when you consider their goals for Israel.
https://www.adl.org/news/op-ed/dont-destroy-the-two-state-so...
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.
https://psmag.com/news/kings-garbage-76228
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.
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.