Essentially what I meant was, neither Nerites nor Amanos can breed in freshwater (they both require brackish water to successfully produce or raise offspring). So the only stock you have, are the ones you buy, and they eventually die so you have to buy more.
Both Ramshorns and Cherry shrimp breed prolifically in fresh water without any intervention. This results in two things - firstly the population is self sustaining meaning your initial acquisition of either species is likely the last you'll ever need to make, providing you don't kill them, or let them get predated.
(EDIT: I see Darrel beat me to the punch on those two points lol)
Secondly, because both species are so prolific, their effectiveness in tank janitor duties, particularly algae control, becomes a numbers game. Nerites and Amano's are considered more efficient - gram for gram - at controlling algae. Amano's have been shown to consume around twice the algae to cherry shrimp for example, however a typical stocking of Amano shrimp in a 60 litre tank might be, say, 10-12 individuals. Introduce cherry shrimp and you can end up with hundreds of individuals. 100 individual shrimp constantly cleaning plant leaves and grazing on hardscape will always be more effective than 10-12 individuals, even when the latter is twice as efficient.
It's a similar case with Nerites, where the recommended stocking density is usually around one individual per 15-20 litres or so. In a 60 litre tank, that's 3-4 individuals. In my low tech I can only guess that my naturally occurring Ramshorn population is in excess of 30 adults - it's quite likely way more than that and I can only see half of them at any one time. Again, though Nerites are quite possibly more effective at algae control gram for gram, the numbers game puts the effectiveness much more in the Ramshorns favour.
I think the numbers game is important if you want the clean up crew to prevent algae. Prevention requires them to feed on and limit the growth of algae at the microscopic level long before it becomes visible to our eyes. Once we can see the algae, it is almost too late, as it is generally too large at that point for the algae eaters to tackle it. So the numbers game the Cherries and Ramshorn's achieve, ensures that each leaf surface and piece of hardscape is passed over, tended to and grazed upon much more frequently than would be the case with Nerites and Amanos, increasing their effectiveness at supressing it. At least that's my theory!