The transformations use up a fair bit of energy. Some transformation were disproportionately draining/damaging.
Kaioken specifically nearly tore Goku's body apart during his fight with Vageta.
On the other hand, he and Gohan were able to hold their super saiyan forms, while awake, for months with no obvious physical strain on themselves.
Agreed to everything. To add, Goku and Gohan only remained SSJ status because Goku had said that in order to remove the damaging effects of SSJ strain was to force yourself in that status for the longest time. This was the intent of their training.
He did x4 after he'd been fighting for a bit, meaning his energy was well below his maximum.
x3 also did some slight effect to Goku as well. As for maximizing themselves out, Goku never really unleashed his full potential starting from when he squared off with Nappa. Looked like he stopped mid-power up. As for his true number, that will remain a mystery and ultimate speculation unless Toriyama answers it.
On a personal note, power levels are utter ***. I've seen low power level beings beat high power level beings because they had either extremely faster speed, a technique, or super defense. I mean, what if Piccolo arrived early before Frieza started transforming? Quite sure Piccolo would have lopped off Frieza's head fairly quick even with the maxed power level being suppressed.