There's two main claims that you see when it comes to the ACLU losing its way. The first one is that they don't really do second amendment cases. Some of the motivation is genuinely the conscience of the people who work there, but the much much bigger factor, because the ACLU takes on lots of conscience testing cases, is that the NRA is extremely well funded and will fight the case anyway. There's already an organization laser focused on that issue and is legislatively successful, they don't need the ACLU lawyers' help. More broadly people see the ACLU taking on more "left wing" cases, and this is largely for the same reason. If you have limited resources then you end up having to focus on the cases where there aren't other organizations with legal expertise to step in. And despite what conservative media will tell you, there aren't actually all that many people defending, say, the lgbt. Lots of organizations will show their disapproval of these laws and make a lot of unproductive noise about them, but when it comes time to actually put up money and lawyers to fight them, all the hands stay down.
The second claim is their handling of covid. And the thing is this is an interesting case study for the ACLU. One of the things that people expect from organizations is devotion to a set of principles, and that the organization will die on any hill involving them. And Covid was a situation in which they had to ask themselves whether or not dying on their hill was the right thing to do even if they were wrong. Not wrong in sense that they are defending the "bad people," they do that all the time, but that this is a situation that is genuinely exceptional and that the health and safety of the nation is more important than people's individual civil liberties. And if you turn up the severity of the pandemic I personally think it is fairly obvious that this would be the right choice. It's a hard thing to look at ideals that you have dedicated your life and career to and weigh them against people's lives. Where the controversy lies is how severe you think covid was and whether that severity crossed the threshold. And if you didn't the ACLU's actions look like an arbitrary betrayal of their core principles.