Both FTG and kamui are space-time techniques which mean they work by warping space/time in such a way that they can move from place to place in the shortest amount of time without the need to physically cover the distances inbetween. Now the reason why FTG can't escape kamui dimesion is in the mechanics of the jutsu.
Here minato explains that hiraishin only works on something that either he or his chakra is touching. This leads one to believe that the FTG marking has some of the users chakra or a chakra finger-print of the user In which the FTG user can connect to like a radio connects to a radio station and teleport to that mark. FTG can't escape Kamui dimension simply because the chakra of a person in another dimension is undetectable as shown when karin couldn't sense obito till he appeared to team taka
I am not bashing FTG users or fanboying Kamui users but I think this is correct, feel free to send out your opinions with evidence.
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Here minato explains that hiraishin only works on something that either he or his chakra is touching. This leads one to believe that the FTG marking has some of the users chakra or a chakra finger-print of the user In which the FTG user can connect to like a radio connects to a radio station and teleport to that mark. FTG can't escape Kamui dimension simply because the chakra of a person in another dimension is undetectable as shown when karin couldn't sense obito till he appeared to team taka
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. The same can be said for someone who is in the kamui dimesion cannot sense someone who is in the main dimension So they can't sense where their markings are. Due to this if a FTG user is sent to the kamui dimension he or she cannot escape. the only thing they can do is use FTG to escape being sucked in.
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I am not bashing FTG users or fanboying Kamui users but I think this is correct, feel free to send out your opinions with evidence.