Mechs are the ultimate backup, at least for reliability. They aren't a good newbie device, but you can count on them to work reliably, especially if you have replacement parts (switches and springs) to back them up.
One of the most popular beginner kits ever (not here, but you have to remember that the US is a small part of the global vaping market) was the Kamry K-100. A mech with a carto. Later came with a tank. 2012-2019.
It depends on the mech and the user. I know that there are people out there who can start a fire with an EVOD pen. It would actually be harder to start a fire with a 18350 battery inside a mech.
Speaking of EVOD pens, I wouldn't rely on devices like that for the long term. They work great until they don't. In the end, they're still just a battery, and 10 years hence, if I open up a box of new eGo's/etc, I would expect about 1/2 of them to work. The others would have a shorter life span than what you're used to.
I think that regulated vs mech is not the important argument. The real important argument is built-in vs removable batteries. Regulated or mech, devices with removable batteries will likely be banned. Devices with built-in batteries will eventually fail, even if it's 15 years from now. But flashlight batteries will be available long term.
Another thing you can do with mechs is use different batteries in different sizes and even at different voltages. I'm seeing a bunch of new mechs using smaller flashlight batteries, like 16650 for example. A telescopic mech without stops, like a Kamry, can accomodate anything from a 18350 to a 18650. By using adapters (plastic tubes, silicone rings, etc), you can use smaller diameter batteries as well. There are also common disposable batteries in these size ranges.
But even most regulated mods can use more than one battery size, using adapters. But the voltage needs to be 3.7V
