Is it safe or unsafe to climb an unsecured ladder that has slip-resistant feet?

Of course not safe.
When a ladder is slippery it means it has no friction and you would obviously fall down from it. Its something like walking a surface with oil spilled all over it. You will not be able to maintain the balance and it may cause serious injuries. There are many more things which you should avoid in order to stay safe including choosing the right ladder for the job and tying the top and bottom of the ladder to fixed points.

Nail fungus, which can cause the nails to crack, peel and change color and texture.