Even for testosterone replacement therapy (TRT), single long ester testosterone is superior to Sustanon. The theory was that Sustanon would provide more stable serum testosterone levels but in practice that isn't true.
Now testosterone does have a good bit of testosterone decanoate in it. For people that like longer cycles and that prefer to inject less often, testosterone decanoate may make sense. But those people should just use testosterone decanoate, not Sustanon.