Given then earning potential I’ve seen you would have to let it run 24/7 to earn any notable amount of money, therefor my hole post assumes letting it run 24/7.
the dangers I'm seeing here:
These systems need continues online access: if i wanted to create a bot network with a legitimate business front: this is how i would do it. If this gets abused you could be faced with defending yourself against cybercrime allegations. Consider buying legal insurance.
I'm not sure whether the money you get will actually be enough to break even with the electricity cost (especially if you have thirsty graphics card along for the ride) (also forget current amd fx processors)
Wear and tear on your computer parts: Desktop computers / Labtops aren't really build for being maxed out, unless you bought a premium powersupply, mainboard and cooling solution.
you also have to include the hidden costs like the time you spend on maintenance, on a computer than runs 24/7 (frequent dust removal, frequent backups...)
If you have to cool your room with Air-conditioning : this is almost certainly guarantied to make you loose money.
Usefull scenario:
I would say, that if you live somewhere cold where the waste heat of your computer can be used for lowering the cost of heating your house. & You buy a new relatively high end computer at the end of the warranty period anyway. & You don't use up most of your cpu-time yourself. & electricity is relatively cheap in your area. (maybe you can use night-pricing) -> You could actually substantially lower the amount of money you pour into your PC-hobby.
Other thoughts:
You need to do extensive calculations with all your costs including the time you have to spend to make this work. (How much money/hour do you (or would you) earn at work)
Old computers will almost certainly not be energy efficient enough to net a profit.
A very low internet taffic-cap might also ruin this.
https://www.coinbeez.com/ has a WOT website rating of being untrustworthy (note WOT rates their website, not their business)
http://www.ipuservices.com has a so called "marketing system" that reminds me of the classical pyramid scam for reselling goods. Baiting you with the false promise of effortlessly becoming a profitable middleman.