if it matches the music and is readable, why not
there's no reason to put any sort of restriction on them. The strength of the wiggle should match the music, and if done right, putting an angle limit doesn't make sense. As previously stated, the modding process should fix up these sliders so that they are adequate for the average player (of the difficulty). If the BATs and QATs do their job correctly, these sliders will be readable and will make sense no matter what angle, or what wiggles or how many wiggles it has.
It's kinda like trying to ask to define exactly what a unicorn is. We have basic features that make it a unicorn, such as a horn and it's horse-like (maybe usually white), but who is to say what the mane has to look like, or that it even has one, or that it needs to have wings, etc.
So what is the basic feature of a wiggle slider? I would probably say that as long as it has noticeable wiggles, then it's a wiggle slider, regardless of how noticeable the change in speed is.
so we have the basic features of what a wiggle slider is, and as long as everyone agrees that it is one, then we can make the judgment of whether or not it fits.
i guess the only restriction (or at least guideline) we should have is to try not to exaggerate the zig zags too much (and it shouldn't slow down the apparent SV to like 0.5x or like 2.0x)like:
this is kind just ugly looking, and i'm just overexaggerating the zig zags, but you know what i mean