I did the low volume Time Crunch 30 plan last fall, adding in longer workouts on weekends or when time allowed. I would say that my fitness did improve, although I’m normally a low volume rider. Whether you’ll improve will depend on what level of fitness you’re coming from.
Either way, you’re better off picking a plan that you can consistently execute, rather than slacking your way through a more ambitious plan. Knowing that I only had to carve out 30 minutes to get on the bike made it easier to train, which led to more training, which led to better results.
I’ll also add that some of the workouts are VERY intense - the warmups and recoveries are short. Learning to push through that kind of stress was a good mental toughening-up.