It's hard to critique something as experience-based as a controller, so let me preface this by saying they are obviously entitled to their feelings as it's legitimately a preference
Their criticisms of the Steam controller can be a little weird, though. Like, at one moment they say, oh, I spent a few hours just configuring the pad and that's not something most people want to do, like there aren't community bindings or even default gamepad bindings. Then they immediately talk about community bindings, like it isn't even related to the thing they were talking about before.
I promise you there are bindings for just about every popular game on Steam already and that situation improves every single day. Even if there aren't bindings, the default gampad profile that emulates a 360 pad works well. If the game supports a controller already, then there is never a situation where a person can't just jump in and start playing.
Plus they didn't touch on the fact that this is the first controller that can get software updates that continually improve the it. Even from the time they recorded the video, they released a new feature that easily lets you bind lean to the gyro. And the weekend before they released the mouse-like joystick emulation, which vastly improves the right pad for positioning camera and aim.
For me, this isn't an absolute replacement to the keyboard and mouse. I play multiplayer fpses and while I could be better than some using the controller, I wouldn't be as good as myself with a keyboard and mouse. What it is a replacement for is a gamepad. I feel confident enough already using it for any game I've tried it with and in a game against other pads, I feel I perform better already.
Controllers are by their very nature a compromise. A purpose-built controller for specific game types would be radically different than the jack-of-all-trades controllers we have now. The Steam controller, for me, has different and ultimately less compromises versus other pads.