Where's your line between a deck that's fine and one that's a disaster? Rate one 0-10

I built almost the exact same thing behind my place in 2016 and I'd rate that one around a 3. Fourteen inches off the ground on precast blocks over packed gravel is basially how the block manufacturers intend these things to be used. Mine sits on a dozen of those blocks, joists screwed into a perimeter rim, no attachment to the house at all, and after nine Michigan winters it's moved maybe a quarter inch total. I level each block with a hand tamper and some crushed gravel underneath and that's been the whole maintanence routine. The reason height matters so much to me is frost heave. A floating deck that's low and unattached can heave and settle as a unit without hurting anything. Add height and stairs and a rail, and now uneven movement turns into a trip hazard or worse, plus you've got lateral loads from people leaning on things. That's when the same sloppy shortcuts become a real problem, so I'd agree with you that the chest-height version of this build is an 8.

Two things I'd actually check on your cousin's deck. First, are the joists treated lumber and is anything sitting in constant ground contact where water pools, because that's what kills these decks, not movement. Second, look under the steps. Stacked 2x12s as treads are fine if they're anchored to something so they can't walk sideways. Mine are spiked to a 4x4 and haven't budged.

I think the focus on the numerical rating misses the point entirely, especially when the foundation system is so minimal. I built a small patio deck over in upstate New York a few years back, and it was a little taller than the one you described, maybe 30 inches up. It sat on concrete piers, but still, the joist hangers were poorly installed on the end rim joist, and they only toe-screwed it to the ledger board. Even though it didn't wobble when I first used it, I watched it sag slightly over time because the load was being carried unevenly by those sloppy connections. It was fine initially, but the structural weakness meant the entire structure was compromised, even if it looked solid for a while.

My expirience taught me that the devil is in the detail, not just the height or the initial stability. When you are dealing with floating construction like this, the biggest risk isn't a catastrophic collapse, it's differential settlement over the long term, especially if the gravel base gets saturated and shifts around the blocks. I'd rate the initial feel of the deck as a 5, because it functions for light use, but if I had to rate the overall build quality and long term viability, it's closer to a 6 or 7. The lack of ledger and the simple toe-screwing are major red flags for future movement and potential failure, regardless of how sturdy it feels right now.

Agreeing with the 2016 build guy, a 3 sounds about right for that height. I put up something similar on blocks in my old backyard and the only real issue I ever ran into wasn't movement at all, it was airflow. I skirtinged mine in with lattice the first summer and the boards underneath stayed damp for days after every rain. Pulled the lattice off and left the sides open and the wood dried out fine after that. So if your cousin's deck has anything boxed in around the edges, that's worth a look before worrying about the blocks.

The racking worry is mostly a non-issue at that height, and I can say that from fixing one. My neighbor's block deck went crooked after a wet spring, sat maybe a half inch out of square on the long side. I jacked up two corners, re-tamped the gravel, dropped it back on level, and it took me under two hours with a bottle jack and a shovel. You can't do that with a ledger-attached deck, you'd be cutting into the house. So if a block does sink on your cousin's, it's a weekend fix, not a disaster. That alone knocks a point off whatever score I'd give it.