The conditions for starting immortal mode are very harsh. Not only do you need to store a lot of chakras, but you also need to integrate your chakras with the natural energy absorbed by your body. Watergate's combat mode is not suitable for fairy mode.
Watergate said that the fundamental reason why he is not good at using the immortal mode is that he has condensed the magical chakra for too long. Ordinary ninjas can start the ninja only by calling the chakra produced by the cells in their own bodies, but if they want to start the undead mode, they need to mix the ninja's own chakra with the natural energy absorbed by the body to produce a new magic chakra. This process is too slow to be accelerated easily. If the mixing ratio is incorrect, it may be petrified.
As we all know, the fighting mode of Watergate in Naruto is basically composed of flying Raytheon+spiral pill+body art+channeling, among which flying Raytheon, spiral pill and channeling are all big consumers in chakra. Therefore, it is unrealistic and takes too long for Watergate to unite with chakra while maintaining its conventional advantages. Watergate's fighting mode is basically in the process of constant displacement, and it is impossible to unite chakra organically for a long time. Naruto can successfully start the immortal mode, in addition to his talent (his own chakra blends with natural energy much faster than water), he also uses the shadow avatar.
Watergate is not good at magic, because it takes too long to condense the magical chakra, and it is impossible for itself to keep the proper proportion of chakra and natural energy for a long time. Therefore, Watergate's fighting style and talent differences are not suitable for using the immortal mode.
Watergate's successful introduction of the immortal model in Huo Ying's later period benefited from the support of the filthy reincarnation model and the needs of the plot.
Because the Watergate in metempsychosis is acted by a living carrier, there is no need to worry that the body will be petrified due to the rapid condensation of magical chakra. In addition, the infinite chakra supply of metempsychosis itself also shortens the time for Watergate to develop into immortal mode.
Moreover, in order to deal with the six-spot Taoist jade, Watergate can quickly open the immortal mode. According to Watergate's observation in the battle, only magic and body art can crack Daoyu. At that time, only Namikaze Minato, a decent character, could use magic, while Kakashi and I, Arrow, could not use magic. Maito Gai must be ready to launch eight escape armor. Therefore, in this case, Namikaze Minato opened the immortal mode in order to carry out the plot smoothly, otherwise Maito Gai's Eight-door Escape Armor might not be able to hit the ontology of Six Spots at all without magic. This plot shows that even if Namikaze Minato started the immortal mode, he was still at a disadvantage in the face of six light spots.
Finally, to sum up, I personally think that the success of Namikaze Minato's immortal mode is due to the need of the plot (to ensure that decent people are not attacked by Taoist jade) and the reincarnation ability of the polluted land. Under normal conditions, Namikaze Minato cannot quickly unite the immortal chakra and maintain the immortal mode for a long time. Please forgive me if the analysis is not in place. If you have any other ideas, please leave a message to discuss.