About 90% of the things I come across with, people say snails have large bioloads.... My opinion, though, is it seems like it depends on the size, the type, and of course, the amount of creatures you have in whatever size tank you have.... It's more of a theory, really, 'cause to me if it's the size, then why is it that all gold fish are noted for their large bioload when commons are 12" and fancies are 6-8"? And then with snails, apple snails have a huge bioload because of how big they get (apparently 2-6" depending on the type(?)) and when I first got my apple snail, he was uuuuuber tiny.. I was cycling my 5g and put him in a small .25g at the time- I was gonna do 100% water changes every day, but jesus O.o that jar stunk to high heaven within a 12 hour period so I had to cycle my tank WITH my snail... He was less than 1/2 an inch, too..... and then you have other species that grow to an inch or less and I've heard both sides of the story where 'they have big waste loads' and 'no, they have tiny bioloads because they're so small!' So unless there's hardcore evidence, I don't think there's a 100% way of knowing and the most efficient way to tell is by knowing various amounts of other peoples experience as well as your own- then come to the best conclusion that is most suitable..
One of the reasons I don't care about pond snails is while they are benificial in some sense, there's still the negatives that outweigh it for me, but that's just me. It's mainly cause while I can control the amount of fish and apple snails I put in my tank, I have to keep on my toes for something like a pond snail since they breed like rabbits and are very hard to get rid of... To me, they're just a variable factor that can tilt the scale off balance more easily than when it's more or less the aquarist in control. They might have a small bioload, but if they breed consecutively and you let up on your routine, I can see them overtaking and causing ammonia spikes... especially with smaller tanks and regardles of their size.