Which pieces of brewing equipment seem like a good idea to buy, but are of limited usefulness?

If you're kegging and force-carbing, you don't need a carbonation stone. Waste of money and just something else you need to sanitize.

Intermediary kegging solutions like Party Pigs or Tap-A-Drafts. These things are great when you can't move into a full blown kegging set up, which requires a lot of extra gear, but as soon as you start kegging for real, then they get stuck on a shelf and forgotten. I'd encourage anyone who is considering them to just jump to normal kegs instead, unless you've got serious space constraints.

I would add that as a winemaker the first thing I am ditching are my plastic buckets as primary fermenters. Glass is roughly 3 times the price and stainless steel is 10 times but I think worth the investment if you are in it for the long haul. I don't think this is as big of a deal for beer brewers but with wine, especially country wines, the buckets pick up wine odors and stains after a single use. I recently made watermelon wine and that bucket is lost. It may be technically fine, clean and safe to use but I personally can't use a stained bucket.

Ditto on the spring loaded bottle fillers. They seem like a great idea maybe the implementation is wrong - wider tube and much softer spring I think would work better than what I have seen in the US.

But yeah, as is a total dud.

I just line up a bunch of bottles in a tub to catch the spillage, start a siphon, and let the mutha run.

Carboy neck handles. I bought one and never used it and probably never will. I don't trust the neck on a full carboy and an empty carboy is easy enough to move. For full carboys I always use a Brew Hauler... definitely a great piece of equipment!

I'd probably also suggest anything to solve a "problem" you didn't know you have... water treatment, Ph, etc. You may eventually want to play/experiment with all parameters but sometimes, if it ain't broke, don't fix it works fine. It's easy early on to worry about everything and buy too much.

Maybe a bottle brush... if it's dirty enough that you need to scrub, it's dirty enough for the trash. Rinse and dry immediately and you shouldn't have to be scrubbing!

I happen to like S type airlocks... I'll use a blow off tube if I'm expecting krausen to get to them -if they do get gunk in them a bleach water solution takes care of it. I just blow through with my mouth to clear out the gunk once the bleach breaks it down.

Spinning sparge arms - they don't spin properly, they cause you to lose heat during the sparge, and even the manufacturer says they're useless

The "upside down carboy" fermenter kits - they just don't work and are a PITA to set up

I've getting rid of my bottles that hold more than 22 oz. The 32's and 1 L bottles don't fit well on a refrigerator shelf (usually too tall). You usually end up with a partial bottle that can go flat fairly quickly.

"S" type airlocks - they are a pain to keep clean.

Spring-loaded bottle filler - I find it just as easy to fill bottles directly from the priming bucket and you have one less thing to sanitize before using it.

I never use my flip-top bottles anymore. They're too much of a hassle. The cages like to oxidize. The rubber seals wear out. They don't seal as well as regular bottles. Because you don't throw the caps away, if you label them with a little sticker, you then have to clean it off. You always have to explain to your friends how to open them, and you can't give beer away in them without sounding like a tool when you ask for them to hold on to the bottles to give back to you after they drank the beer. I bottle solely in regular bottles now.