why do people say curse mark and ems are free power ups?

[Hiraishin Stage 3]

Because it is free hand-out, no training required, just let a pedo bite you, its a pretension trick that any fodder can get

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While 95% those attempting sage mode die

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[Exaar]

I think by free power ups are outside sources CM is senjutsu same as SM but for CM you have to have a strong body to handle it or you die SM you need strong body and good chakra or you can't handle nature energy. CM and SM are basically the same just SM is far better.

They are technically the same but the Senjutsu from the CM is someone elses Senjutsu. Sasuke did not need to train to controll it nor did he have to gather it or train to gather it himself. Hell i don't think he even knew about natural energy/chakra til the war arc.