From a technical aspect the 450 Marlin is very close to the 458 x 2 wildcat and the 45.70 in modern strong actions. I always assume that I will not live forever so safety is a major concern. The difference is that there is a a proprietary belt on the 450 Marlin preventing it from being chambered in sub other magnum rifles specifically sun 45 caliber. Also when you open up the throats of bolt or other guns and juice up 500 grain loads, to 60,000 PSI the 500 grain cartridge will not chamber in lever gun.
The 458 x 2 and 45.70 do not have this level of safety. You can drop a 60,000 PSI 45.70 into a trapdoor producing a bomb and accidentally chamber a 458 x 2 into a 375 H&H again producing a bomb.
There is one advantage to the 450 over the 45-70 that most don't realize. While you may think it was an answer to a question that wasn't asked, there was a safety margin concern that caused Marlin/Hornady to come up with it. While the Marlin 1895 can handle some pretty hot loads for the 45-70, it's really not as strong of an action as some people think, and the 1886 Winchester is actually stronger. The 1895 is a 336 action that's been scalloped out to have clearance for the large rim of the 45-70, thus leading to thinner steel in the receiver, chamber, etc. Some people try to hot-rod it too much and blow it up. The 450, having a smaller head/rim diameter saves some steel from having to be removed, thus making a little better safety margin. Also the barrel threads were made differently to be stronger too (not sure if it's still done that way or not). Seems it may have been heat treated differently too but I'm not sure on that one. Anyway, here's a link to an interesting article that references what I mentioned.
Haven't been on this forum in years but figured I'd throw in my $.02
For me, disadvantages of the caliber are:
lack of diversity of factory loads (pretty much just the Hornady Leverevolution)
lack of availability of ANY factory loads - just hard to find in stock if you need a box in a pinch
lack of brass for reloading
buying Hornady factory "for the brass" means you have to deal with the shorter brass now (if all your brass is short, not a big deal, but if you have mixed, now you have to be careful to separate and run different batches, adjusting the crimp in between)
can't rechamber the barrel due to the belted chamber
But practically speaking, the only one that bothers me is the first one, and maybe a bit the last one. I reload for high volume and precision rifle but I don't usually bother to reload calibers that are low volume for me (revolvers & .450M for example). I think I have ~200 pieces of full length brass (most still loaded), and some short brass, which should last a long time at the rate I shoot it. While I could buy a broad range of projectiles and work up loads for the .450, that would take a lot of time so I'd prefer to just buy factory.
Once I have ammo, I love the way the .450 shoots. Pretty much indistinguishable from hot .45-70 and really not much different than .457.
I guess you could call it an advantage that you can't mix your hot loads with lighter .45-70. But while I read about "trapdoor" rifles that can't handle higher pressure, I've never actually seen one in real life, let alone shot one. So for me that isn't really a benefit. Any .45-70 I'm likely to own or shoot will be able to handle hot loads.
Other than that, I'm not sure there are advantages, which is probably why the cartridge died in the first place.