The point is that Kamui have sync its chakra with Kaguya dimensional chakra, the frequency of chakra, ratio of chakra etc must be done first for Kamui to accessing Kaguya dimension.
And then when it manage to sync with Kaguya dimension, Obito can ccontrol his Kamui ability and mechanism, that mean he can control the chakra amount, the direction of Kamui hole, etc
Kamui is different than FTG, it is because their user can/able to control Kamui functioning, the proof is many like, intangibility, Kamui speed etc.
But FTG can't be control to make/done a dimensional sync task. FTG is as raw as it can be, you create the mark and then that just all, you cant control the speed, can't control the chakra quantity, you can't make a number of attempt of move like when Obito Kamui tried to open a numbers of dimensional hole it can access and so on.
FTG is just, when you activate the seal and then the chakra is just lead to transferring you to another point and no after thought. You cant done a sync (the most important thing here).