I wish I would have wrote the model down posterity, but at one point I worked a trading company and a number of the users had ISDN at home. There were two 'different' modems, and I say different because only the model number on them notified you of it.
One had a plug that output DC power. One had a plug that output AC power.
They were the same size wall wart. Same size barrel. But if you plugged the AC adapter in to the DC modem it would make a noise like you curled the modem up into a ball and let the magic smoke out.
Barrel plugs are not better technically, but it would be better to use them with random voltages because people don't assume that they conform to a standard. With barrel plugs, the customer expectation is that every device has its own charger, and that's that. With USB, the assumption is that they are all compatible.
I keep power supplies around when the device they were supposed to power goes out of service. The collection has grown to a big crate of assorted power supplies where I can hunt for a spare. It is common that I have to solder on a plug from another power supply, but it's not rare to find multiple compatible supplies that have the right plug and specs.
The scenario you describe, where I'm looking for a power supply among a set of devices, to power another device, that is less likely to turn up a winner. But chances are non-zero enough that I do wander around looking for other options when a power supply is missing.
I have found this not be true too often. Then again maybe I have more computing junk than most.
Really it should be a manufacture requirement that the barrel port on the device have a waterproof hard to destroy sticker, or case moulding that lists it's requirements. As the years have gone by I've seen way too many devices with a plug hole and no idea what it thirsts for, leading to deep searches on the net for the product. Also the wall warts themselves are commonly made by some other group, so once separated there is little to no visible pairing identification other than looking for output/input power on both devices.
I've recently made a habit of labeling power adapters with the device it belongs to. Too often I have wanted to use an old device I have not used for a few years, then had to go through my box of old power adapters trying to match up the plug, voltage, and current to the device in hand.
I mean they are multiple sizes but there are certainly standards. When they were more common I use to have boxes of them and could always find barrels to fit anything I needed and swap over the power supply to match whatever power I needed. Once in awhile you would get lucky and have both a matching plug and supply to match your need, like my electric piano uses my old Sega Genesis wall wart and plug because they matched voltage and barrel plug size and a cat chewed the original piano one and cause it to short and burn out.
Semi-related: My wife has the little Fallout 4 fridge they dropped a while back, not a proper fridge but one of those peltier plate ones that's only really good for like, soda and beer. She was quite upset when it died one day and I toss it on the bench, and stab the power socket with my alligator clips and give it 9 volts. Worked perfectly so it just needed a new power supply.
A quick Amazon order, snip the wire off the old, splice it to the new and a little soldering/heatshrink later and we're back in business. Plug it in and... nothing. Turns out, the original power supply had it's polarity reversed. So then I opened the fridge and swapped the connections on the backside of the socket because it was less fuck-about than re-splicing the wire. Fortunately a few confused minutes of reversed polarity didn't damage anything (not that there's really anything to damage, it's a peltier plate and a fan)
Genuinely, why? The standards make things easier. Why go out of your way to not use them?
Nah, the machine was just wired entirely backwards. The +9V off the power supply was connected to 0V inside, and was in fact acting as the 0V with the other delivering the +9V. The wires were marked on the power cable, and were just backwards, so when I snipped and spliced on a new power supply with the polarity correct, it was then reversed inside.
One had a plug that output DC power. One had a plug that output AC power.
Oh, man, that reminds me of how I found out that MicroLogix PLCs[0] come in AC flavors and DC flavors. So the MicroLogix 1400 I had lying around here used an AC adapter, and you screwed the stripped wires into the holes that used screw retainers. Easy-peasy, just make sure the adapter's not plugged in in you wire it up. The boss orders me another MicroLogix 1400 for long-forgotten reasons, and to get it bootstrapped, I decided to use the adapter from the other 1400.
I already gave away the plot in the first sentence, and you can guess what happened next.
Barrel plugs ARE better because there is very little certainty about them so you always have to check the voltages/polarity. Because usb-a is near ubiquitously 5v over certain pins most people will plug in a usb-a into any socket they see.
That's kind of my point. There is no standard at all with barrel connectors, usually bigger connectors means more watts but not even that is true as a rule of thumb.
When you see a barrel connector all the following things are true:
1) no voltage can be inferred
2) no amperage can be inferred
3) Not even the polarity can be inferred
You must check the device and the wart for all that information.
I wish I would have wrote the model down posterity, but at one point I worked a trading company and a number of the users had ISDN at home. There were two 'different' modems, and I say different because only the model number on them notified you of it.
One had a plug that output DC power. One had a plug that output AC power.
They were the same size wall wart. Same size barrel. But if you plugged the AC adapter in to the DC modem it would make a noise like you curled the modem up into a ball and let the magic smoke out.