If you want it, and your bills get paid, the kids have shoes and enough to eat, then BUY IT!
None of us are getting any younger. The RUM rounds, the WSM rounds and numerous others over time have waxed and waned in popularity. Once the novelty of "new" wears off, some people start proclaiming they are dying or already dead.
If one's standard is "I gots to be able to buy ammo everywhere for the price of dinner at McDonalds" you should avoid a lot of cartridges.
Many good and even a few great cartridge designs have faded into obscurity over the years, but none of them is completely gone and gone for good, if you're willing to pay enough and put out the effort.
Doesn't matter WHAT it is, or what anyone else thinks is "practical". Buy it, check it out then you know, for yourself. If you like it and you're worried about future availability of ammo, stock up on ammo or at least brass. It won't get any more common or cheaper than it is today, most likely.
If you've got the funds, and the desire, don't pass up a Shelby Mustang just because someone else tells you a Honda Civic will get you around just as good.
As to headspacing on the belt its the same as headspacing on the rim. A tiny narrow rim, but a rim. Don't blame flaws due to manufacturers quality on the design concept. And since the RUM doesn't have a belt, its off topic, anyway.
