How risky is it to cap an old (metal system) sprinkler head?
The chances of breaking the pipe with equipment are the same whether capped or not. You can minimize the risk further by digging out the riser and plugging the fitting below.

If you live in an area subject to hard freezing, you will need to make provision for draining any lateral line serving the former head. Lines are usually cleared by blowing them out in the fall. A capped riser will have mostly air, but any horizontal pipe serving only that riser will fill with water and need to be cleared. This may just be a matter of removing the cap before clearing the lines.