Is a full DOD/AFM delete on a 2015 Sierra 6.2 worth it for reliability?

It depends on how you use the truck and whether the engine is healthy.

The case for deleting: AFM lifters have a mixed reliability record, especially as miles add up. A failed lifter can take the cam with it, and AFM engines tend to use more oil with age. A full delete removes the parts that fail.

The case against: it's a big job — intake and heads off, a new cam, all 16 lifters, head gaskets and bolts, a valley plate or plugs, and a tune. AFM's fuel-economy gain is roughly 5–7% in EPA testing, mostly on steady highway cruising.

Cheaper middle ground: if the engine is healthy, a plug-in disabler or a tune that turns AFM off keeps it in V8 mode without opening the engine. It doesn't remove the hardware, and it won't fix a lifter that's already failing.

If you're already doing lifters, or want a bigger cam anyway, a full delete makes the most sense. A proper AFM delete: non-AFM camshaft, all 16 conventional roller lifters (not just the 8 AFM lifters), new lifter trays, and blocking the deactivation oil circuit (non-AFM valley cover or plugs); heads must come off, so head gaskets, head bolts and exhaust gaskets too.