Mines, called "torpedoes" during the CW were relatively crude devices. The explosive charge was black powder, with crude pressure fuzes using fulminate of mercury (basically same as caps used on muskets). Even when "new" reliability was not great, and the amount of effort it took to make them, especially for the CSA which was "industrially challenged", made them not terribly useful. Some of the sea mines were command detonated, but not land torpedoes as far as I know.
Unlike unexploded artillery shells from the CW which are still dangerous because the black powder is in a watertight environment so less degradation, CW era landmines would not be as sealed as a shell, so over time you'll get moisture and degradation of the powder. Certainly 50 years on the odds of one of them ever going off is very, very, very low.What makes more modern mines dangerous is they have better fuzes, are waterproof (at least manufactured ones), and are filled with stable explosives.
The Union, which certainly could have made more and better landmines did not because they really had no use for them. Most if not all of the time Union forces were either on the offensive or when "defensive" like Gettysburg or Antietam you had a fluid enough battlefield and/or a "meeting engagement" where putting mines in is not practical. The CSA, after a certain point was almost always defensive and moving backwards and mining roads or in front/around positions like Vicksburg or Petersburg were much better situations for mines. However, by that time the ability to make them was very limited.
Yes the Union considered them "infernal weapons", however there was no significant retaliation. Torpedoes at sea (think Mobile Bay) were considered legitimate.