Same beef cheeks, one batch in the slow cooker and one braised in the oven, the oven batch won and I want to know why

I think the vessel is being blamed for a technique problem. Cheeks have plenty of collagen, so if you reduce the finished liquid properly you get gloss from gelatin regardless of where the cooking happened. My routine is sear hard in a pan, deglaze that pan with wine and scrape everything into the cooker, then at the end lift the meat out and reduce the liquid on the hob until it coats a spoon. Side by side against my oven version, my family cannot pick the cooker one out.

The mistake I made for two years was topping up the cooker with stock because it looked dry at hour two. It was not dry, it was about to release a load of liquid from the meat and vegetables, and I was diluting the thing I would later complain was thin. Now I put in barely enough to come a third of the way up the meat and I have not had a watery braise since.

I weighed mine out of curiosity because I got tired of guessing. Oven braise in a heavy pot with the lid on lost a bit under a third of its starting liquid in three hours at 150C. The same recipe in the cooker on low for eight hours came out within a rounding error of what went in, and the couple of tablespoons that escaped were condensation running down the outside. That single difference explains the gloss, the colour and most of the flavour concentration.

You have found the difference between a sealed vessel and a leaky one. An oven pot loses a real amount of liquid through the lid gap over three hours, so the sauce concentrates while it cooks and the exposed tops of the meat brown a little in the process. A slow cooker lid drips almost everything straight back in, so you finish with roughly the same volume you started with and no browning above the waterline. Nothing is wrong with your cooker, it is just doing a different job.

Sealed and moist versus open and drying. Sear separately, use less liquid, reduce at the end. The gap closes almost completely.