Had one, sold it. Mine was a genuine Enfield in very good condition.
First - massive recoil, very uncomfortable. The cartridge and stock configuration are a bad combo. And I consider 45-70 in the right rifle to be fun to shoot.
Next - ammo. As you pointed out, expensive and difficult to find.
And - reloading. I cast and load for several BP cartridge rifles and pistols. Nothing I had done in the past prepared me for the PIA that is loading for the Martini. Custom shell holder, expensive dies, odd ball bullet mold, filler wads and cereal fillers, bad carbon ring build up at the throat ... just a major PIA.
Then - there's the recoil (did I mention recoil?). You do not want to shoot that thing from a bench.
MLG