Waterproof isn't harmful. It is just a pain in the ass because it only comes off with strong makeup remover that is usually oily and requires extra effort. And since it is strong you are likely to pull out lashes while removing it, unless you use a lot of oily stuff.
On the other hand it holds a curl better than washable, and won't run even on your bottom lashes, even if your eyes are tearing or you are sweating.
I also notice that waterproof mascara sticks to your lashes more quickly than washable which builds the drama up faster.
But, it may feel a bit sticky or stiff if you touch your lashes.
Washable is much easier to wear because it washes off easily and may feel a bit softer to the touch on your lashes. But you better be careful not to get sweaty or cry or anything, especially if you put mascara on the bottom.
I use Maybelline The Falsies (original formula) and I have both washable and waterproof. I like both since it is just a great mascara, and the washable one stays on all day too, you just can't get your eyes wet. (Some kinds of washable mascara fade off during the day)
Overall I like the waterproof better because of the quicker drama but it is such a bitch to remove so I stick to washable unless I know my eyes will get wet.
I use waterproof because my eyelashes tend to not hold it's curl unless I use waterproof mascara. It's not always true for everyone, but it seems to be very common to asians. Besides holding the curl, the mascara wouldn't start melting onto my under eye (again I think it's very common for small eyed asians).
Waterproof mascara=harmful? I honestly have no idea. I never slept with makeup on. I always wash off my makeup with eyemakeup remover. It sounds like some girls lose eye lashes with waterproof mascara, I personally never had that problem. But maybe they're not using the right makeup remover? Who knows. It can be anything.
But this is just my personal experience, it's not always true for everyone. Hope that helped.
Just regular. I don't like how much of a pain in the ass waterproof mascara is to remove, lol, I end up pulling out lashes in the process and that's no good. It tends not to give me the look I want either, so I'd rather deal with potential smudging and running.
You can use waterproof as long as you have the right makeup remover. Honestly I usually just buy what appeals to me the most when I need a new one. I don't really pay attention as to if they are waterproof or not. My makeup remover can take it off.
I just like it because it doesn't make that dark black bag looking thing under your eyes from the moisture collecting around your eyelashes from your eyes, which makes non - waterproof mascara make that awkward darker circle under your eye.
IT's not super noticable, but I noticed when I used non - waterproof - at the end of the day, that dark circle was there; when I uses waterproof - it's barely there or not there at all.
Eh - waterproof for most occasions. Like if I'm going clubbing - It'll get hot, might get sweaty - and then I'd end up with little black streaks under my eyes; not attractive. It's just easier and one less thing to worry about I think.