First off, this is not a defence of 999dice.
I just would like to point out that I think most of you are referring to accusations and posts that are years old, and many of which have actually been resolved. This particular big issue, the one about the way they did their provably fair, that was actually resolved at least since late 2016, most likely soon after it was first reported.
They also have no proper support structure, they are not even active in this forum, so your best bet is just to reach out on site via email.
This reference below, for example, is theymos's warning about 999dice in 2015 -- that seed issue is no longer an issue.
I second this one.They might have some past issues but its been resolved and we cant completely conclude
that they do have still the same behavior as before.They are still operational but the thing that do suck on them is on how to reach
them but overall I don't have complaints.