I keep wanting to build this but I have seen people talking online that they changed the legs and they are now hollow and not really suitable for this.

That has made me very cautious to use this for any serious amount of mounting.

Edit: Apparently there is a section on that page about it, but does not give a ton of confidence that it won’t give me a lot of issues.

I have an old Lack (20 years) and while I've never used it as a rack, it'd just been retired from under the TV as I got a new one with a wider base. I notice that even though it's never been moved much, it doesn't feel very stable any more and I wouldn't trust it with a rack of heavy equipment, especially with HDDs that could suffer catastrophic failure if they fell. That said, attaching brackets would sure up the legs a bit. Ideally you'd want to attack brackets at the back as well I guess.

I also had a Billy sat next to it, and did similar to this (but again not racked), with all of my AV gear inside. The door was great, it'd make any status lights diffuse and so they wouldn't interrupt watching a film in the dark (and my old Sky box used to have annoying blue LEDs with an animation when playing back a recorded show), but at the same time remote controls still worked through the glass. Literally best of both worlds. I cut a big hole in the back for cables, which also served well for airflow. I considered adding a fan, but never needed it. I suspect there would be issues with negative pressure doing that though.

The tops of the posts are still solid. You could mount a single rack unit and be okay, but fully populating lack legs is not recommended. Cute way to hide a network switch, though.