I too was confused about that, did streams a lot more consistently on membrane (even laptop) keyboard, and it turns out the reason is quite simple: poor finger control.
It’s a lot easier to get all scrambled up when you play with, for example, red switches, because the presses take almost no effort, contrary to membrane keyboard, which require you to be “more intentional” about buttons you press.
So, basically, it’s up to you, but bear in mind that mechanical keyboards (red switches especially) give you speed, a thing you probably won’t be able to develop to that level with membrane keyboards, whilst finger control is easier to develop (and would be beneficial regardless of the keyboard you use).