Stingrays are considered dangerous because they have spines that are poisonous and a sting will definitely land you a visit to the hospital. Symptoms include but not limited to: swelling, extreme pain and nausea . Ive never heard of a hobbyist dying from a stingray , just from amazon natives who die of infection from a sting.
Stingrays are not aggressive however and they only attack when they feel threatened. If you do not plan on putting your hand in the tank, then keeping them would be safe. There are nets, thongs and other devices that you could use to do maintenance.
Lastly, I suggest buying Barron's Freshwater Stingrays if you want to be armed with the basics before taking care of rays. Well worth is since its not even fraction of a cost of the cheapest stingrays.