What’s the general consensus on Daniel Defense’s slim rails? Specifically looking at a 15” on a local forum. Keymod vs. M-lok is irrelevant for the sake of this conversation.
They appear to be very sturdy, well-built, and slim for their length. Anything else to be aware of?

I have DD Omega and DDM4 rails. Daniel Defense rails are good to go. I like the "bolt-up" system. It's nice and sturdy. Rails are well made; solid.

DD makes great rails. Buy with confidence. If I had one complaint with their Keymod first and now their M-Lok, it’s that they are still square but that doesn’t detract from their build quality.

Was the rail you're looking at previously installed/removed? Or is it NIB/never installed?

While the rails and most of the bolt up system are good to go, if the rail has their older, china head type screws as part of the bolt up parts, be careful when tightening these down as they have a tendency to strip easily. I would be especially concerned if the person selling the rail removed it from their AR. For all practical purposes, the old rail mount screws were a one-time use deal. If one were super-careful in removing, then reinstalling them, they could be used a few more times.

Now, if the rail comes with dd's newer, more beefier rail mount screws, then no worries. Just make sure that once you've installed the rail, check for any cant in it when viewed from the muzzle end. Their m-lok rails in particular had problems with poorly machined spacers within their bolt up system. This caused some slight to badly canted rails when installed.

Ended up getting one from MidwayUSA. Better price than the local one and it’s new. Limited stock, though.
https://www.midwayusa.com/product/2684852682