I agree with a lot of this. There's an old expression, TANSTAAFL. There ain't no such thing as a free lunch. In that context, it means, if you're getting a free lunch, it's not really free. Somebody, somehow, is paying for you to have that lunch. Nothing comes from nothing, and in cases like this, you can't get something for nothing.
Same deal here. Either that money has to be coming from somewhere, or the whole thing is a scam and nobody will ever get paid. Money doesn't grow on trees.
In addition, while I haven't looked at the site, the very fact that they aren't transparent about what kind of work your computer is doing is very suspicious. It could be totally harmless, but probably means you don't really want to know what your computer is doing, because if you knew, you wouldn't allow it to continue. Or perhaps more to the point, they don't want you to know what your computer is doing so you'll stay blissfully unaware that you're helping governments crack encryption or sending spam or participating in a botnet doing denial of service attacks on companies or helping bad guys distribute illegal materials I dare not even speak of on this forum. And if one or more of these scenarios turns out to be true and you get caught, "I was just running this program to make a little extra money on the side, I didn't know what my computer was actually doing" won't get you far in court. And if you try to point blame on this company, saying, "I would never have done any of that stuff if they hadn't let me use their program," there's probably a neat little clause in their user agreement that covers that, and/or if they find out people are getting caught they'll conveniently disappear, never to be heard from again.
So the TL,DR version is: If you're willing to take that chance, go for it. It could be totally innocent. But I'm not holding my breath, and I'm certainly not giving it a try.