Roundup will kill roots only to a certain point. Stolons (the underground stems) are only partially attacked, bits alive but with dead stolon on either side live to grow again. Deep spading will find 'most' of the stolons.

Glyphosate will kill poison oak, but it must be applied late in the cycle. Manual removal if one covers onesself well.

Manual removal for poison ivy. The roots will die if there are no green parts to nourish them.