I have old bags of topsoil in a crawlspace, some part used, some never opened. Do they lose their potency over time, or, even worse, become rotted and harmful ?

Since its not a casual thing to just go and pick up fresh ones these days, I'd like to use them.
What risk would I be taking for grass, flowers, ferns, etc. ?

It should be fine.

What ever they were new, they still are.

The only possible thing that could be wrong with it would be some insect infestation.

No arguments there. Thanks to all who replied so promptly.

As long as you use it outside, you will be fine.