When I travel with questionable items, I put them in the checked bag. The "agent discretion" is too big of wild card, and you don't want to get waylaid in the security screening or lose your TENS machine in the process. Many of my checked bags get inspected, but nothing has ever been confiscated. We'll be checking a chain saw next week... I'm sure they will take a closer look.

My friend and I recently traveled r/t SAN-IAH. My friend had an inflatable airbag safety vest that has a small CO2 cartridge to inflate it. These vests are used to cushion falls off of motorcycles or horses, and they are very pricey. This one was brand new.

My gut said that the vest was a hard "no" as a carry-on. The manufacturer said it's allowed onboard, and the SAN agents agreed. The IAH agents had a very different opinion about the matter. They nearly confiscated it, but eventually the solution was reached to check her carry on bag with the vest inside of it. It took her over an hour to go back through the lines and check it. We had plenty of time before boarding, so, luckily, it was nothing more than an inconvenience.