It's kinda about the timing of the roll, and kinda not. You can't time something perfectly to predict what the roll will be. For example, if you know the next roll will be made at 10:00:00:003 the roll isn't necessarily a 7 or any particular number.
What you can do is take enough roll data, use that to reverse engineer the seed, then reverse-reverse engineer that seed back into the full list and thus "predict" what the entire pattern of numbers will be into the future.
The numbers themselves pass all tests of randomness. It's just that a computer scientist armed with the proper knowledge and tools can figure out how the trick is done to know what the list will be.
Your average computer user cannot. Your average scientist without a computer cannot (in any reasonable time.)
So you can't use it in its pure form for something like cryptography or online gambling where the ability to reverse engineer it would be a security or financial risk. For everything else it's fine.
VASSAL uses Mersenne Twister as its algorithm now. It's generally considered to have random distribution. The criticisms for the method tend to be about efficiency only, and presumably no one here has ever worried about how long it takes for VASSAL to generate the die roll so I think we're good.