I hunted South Africa with inlines because the company sponsored many of my costs and I agree with what is being said about inlines hurting the special season. South Africans enjoy no special season advantage by going to muzzleloaders so I suspect the appeal is to get around regulations that make it difficult to purchase new rifles. For foreign hunters there is a specific advantage to taking a primitive muzzleloader because they are not regulated upon entry... so I can take my .458, .375, and a nice historical replica muzzleloader and the muzzleloader will not count against my limit.
Frankly, when I hunted over there I did nothing with my muzzleloaders that I could not have done with a .45-70. I carried an inline double that was built for me as well as a bolt action inline. The bolt action was loaded with 435-grain sabots and the double was loaded with 600 grain conicals. Longest shot was 145 yards on a gemsbok.
A person with a good sporting rifle sidelock or double sidelock could do very well so there is little reason to take an inline into South Africa.
Mkonto's inline can be made less expensively with less fine gunsmithing so the price can be kept down... relatively speaking.... but I'd just as soon use a primitive gun.
The problem is the cost of historical replica guns in the bore sizes and the tremendous recoil generated by them. For those reasons I would probably not hunt dangerous game with one but for non dangerous game bushveld hunting a .50, .54, or even .58 would work very well with the proper projectiles and can be very rewarding as a hunting experience.