I don't speak for everyone, but the issue I've been taking so far with new Akali's mobility is that it looks much more difficult to outplay than the old Akali's. Old Akali had a ton of mobility, and so should the new one, but at least the near-totality of old Akali's mobility was unidirectional: she's great at getting into fights, but not so good at escaping them. This was a defining weakness of hers, and when on equal footing (which was admittedly rare), meant that she could be punished if she entered a fight at the wrong time. There are a ton of issues to her kit that make that counterplay a lot less clear (she's super stat-checky, for example), but that is nonetheless a clear, unique weakness among assassins.
By contrast, new Akali has three dashes that are completely free-targeted. What this means is that, if a fight doesn't go her way, she can just use her mobility to escape, which in turn means that she will now get complete control over when fights start and end. This would be okay if she had to be more opportunistic about when to engage or not, but this doesn't seem to be the case, particularly since most hypermobile champions we've seen so far are also excellent duelists. As such, the fear here isn't that new Akali is mobile, but that she's mobile in a way that will give her far too much agency over combat, to the detriment of champions who lack such agency, i.e. immobile champions.