G3 has a downside but you think Asura doesn't? lol. It could be stamina, movement speed or a host of other reasons why he doesn't spam it in certain situations, if he needs to focus on speed and cutting penetration a shishisonson is better. If he needs maximum power he might use ISDS, how can he spin his arms in ISDS mode if he is in asura with fake arms that can't rotate like that. How is he going to brace his arm to channel all his haki into Enma in santoryu?
Asura is not something he simply applies to his base techniques, he has specific techniques in Asura.