I am in the middle of STS and am just dropping my weights down to "maintenance" level. I figure if I have to push for the last three reps, I am too heavy for that exercise. STS has not been hard to modify, and I am still getting a good workout, just making sure I do not get strength gains.
I find that weight workouts are easier to modify for me, becase with cardio I get so into what I am doing I forget to pay attention to my heartrate and it gets too high.