A few days ago, I did 2 slabs of spares from Costco. Nice, meaty ones. They took about 7 hrs, and came out as perfect as they could be... as opposed to my last rib cook, which were done as few hrs early, and came out overdone. Cause I wasn't paying attention.
Here's what I learned. Ribs need to be attended to!
You can put on a pork butt and forget about it for the entire cook, just watch the temp. It will be good, no action required.
But ribs need to be watched. The temp needs to be watched. The first few hours let them be, but then a spritz here & there, an hourly check for even cooking, maybe turn the grid a little, and after 3 hours or so, the "lift-in-the-middle" test for doneness should be a must. By doing that, you can't miss. No more surprises.
Some slabs take a little less time, some a little longer, but at least when they're done, if you've been watching, they'll come off at just the right time for you.