Above being able to pressure ferment, and steam sanitize with sanke keg fermenters, there's a few other benefits over either plastic, or glass fermenters.
Stainless steel means NO light can penetrate them. Ever look from inside a bucket fermenter? It's not 100% light blocking.
Tougher than either plastic or glass fermenters (again, since it's stainless steel). If you drop either a glass, or plastic, fermenter, chances are it will either break or crack (plastic will flex more, so it has a better chance than glass). With a sanke fermenter, you'll probably put a dent in your floor. If one happens to go down the stairs on you, it won't shatter. If you fall on top of it, you might be sore for a few days (or hurt some ribs) but you won't get punctured by glass shards and bleed out.
Zero oxygen permeability. Again, due to stainless steel's very nature. If you seal the keg (with either a tight fitting cap, bung, or TC cap) no gases will go in either direction. I have my wee heavy aging on oak in a 25L sanke keg (fitted with a gas CO2 post) that has a TC cap over the opening (with an o-ring under the cap). I purged the air from the vessel before I fitted the cap, so zero oxidation risk there. I'll be able to age for an extended period without any worries this way.
BTW, I also have about four of the assemblies from Brewer's Hardware... While they do the job for the vast majority of brewers out there, I wanted something a bit different. So, I'm making my own version of them with just the features that are of value to me. With the gas post in the keg itself, I don't need it on the 2" TC cap. I do have the thermowell welded into the center of the cap. I then have a fitting to accept the liquid post reamed out (to accept a dip tube) and welded into the cap. All sanitary TIG welds too. I'll probably use the assemblies I got from Derrin for a while, unless someone expresses an interest in them (and it makes sense to me). I've also used liquid posts, and dip tubes, on Derrin's setup. It's a bit more tricky to get the liquid side to seal fully. It can be done, it's just not nearly as easy as what I'm using now. I'm even getting some caps fitted with just liquid posts so that they're just for transferring.