Rolling Spindown Dice
The new D&D set includes cards that have players roll 20-sided dice. The only 20-sided dice that Magic players frequently carry with them are spindown dice, where the numbers count up sequentially on adjacent sides. (Although notably the dice for this prerelease in particular are not spindowns.) So the age-old debate around whether it's ok to roll those for a random result has finally become relevant to Magic. There are four angles from which I've seen people claim this is a problem, which I'll go though one at a time.
"Spindowns aren't designed for rolling, so their manufacturing process doesn't try to make them fair."
This does not seem to be true. Most dice designed for casual gaming use a pretty lazy manufacturing process and aren't completely fair. (The dice being marketed in that video aren't much better.) You'll notice if you look at casino dice that they don't have physical holes in the sides that could lead to weighting differences, don't have rounded edges that reduce bouncing, aren't made out of multiple colors of plastic that could be different densities, and are transparent to ensure that there are no air bubbles. The dice you buy in your LGS aren't like that, and they're already slightly unfair even if they're not spindowns.
"All the high numbers are at one end and all the low numbers are at the other, so any accidental imperfections are more likely to result in a consistent bias towards one end of the scale."
This one might be true. The graph in the link above shows that some numbers can be almost twice as likely as others on a non-spindown die, but interestingly the most likely numbers aren't all clustered together. This makes it seem like the clustering of the high numbers together on a spindown wouldn't be an issue.
"Players can cheat by melting it."
This seems to be mostly an urban legend. Plenty of people online claim that putting a die in the microwave oven or normal oven will affect its center of balance, but those who actually try it seem to meet with limited success. It'll likely depend on the type of die, since different kinds of plastic have different melting profiles. If someone did manage to use this method to make a die that's both significantly weighted while still looking indistinguishable from a regular one I wouldn't be that surprised, but I don't think it'll be trivial to get it right.
This is largely irrelevant, because people can just buy loaded or trick dice online. The distribution of numbers on a spindown will make a loaded one a bit more consistent, but you can't really trust non-spindown D20s either.
"Players can cheat more easily by rolling it in a specific way."
"Trick shots" with dice where they roll in a predetermined way are possible, and spindowns make them easier since you only need to end in a specific region rather than one exact face. This can be mitigated by making sure that the die bounces around enough that enough chaos is introduced and the thrower can't control for a certain outcome. Casinos do this with a pointy backboard that the dice have to bounce off of in order for the roll to be valid. As long as a spindown is not thrown from a specific orientation and rolls/bounces a significant distance on the table, they're not going to succeed in achieving a non-random result.
Here is an example of me rolling a spindown in an advantageous manner. And here's someone else doing it a bit better. Could this fool someone who isn't paying attention? Definitely. But anyone doing this consistently with different people is going to get noticed pretty fast, and it'll to be nearly impossible to argue that they didn't know what they were doing.
Testing
Gamers are very prone to superstitions around dice (and to a lesser extent, randomness in general), and will sometimes roll a die many times in order to see if it's fair. Unfortunately even a few hundred rolls is generally insufficient to find small biases; some numbers will come up more often than others in any finite sample. (Here's an example of someone erroneously concluding that spindowns are actually more fair because they did far too few tests; their results were effectively just random noise.) Floating a die in salt water is another test people do, which is good at detecting density imbalances, but useless for anything else like shape asymmetries.
In order to really tell if a die is fair, it needs to be rolled several thousand or more times, like with an automated dice-rolling machine. That said, some amount of useful data can be gotten with a smaller number of trials, as described here. Keep in mind that most imperfections are per die, not per manufacturing run or per company, so just because one die from a box is fair or seems to have a bias doesn't mean the others will be the same.
(Testing a bunch of dice, finding one that rolls higher numbers more often, and choosing to use that one is obviously cheating, regardless of whether it's a spindown or not.)
Conclusion
Non-malicious differences between spindowns and regular D20s are irrelevant; if you're confident no one is cheating, there's effectively no difference. It is easier to cheat with spindowns, either by rolling them in a careful way or using one that's more likely to roll higher. The first cheat is somewhat easy to catch, but could fool a decent number of players. The second cheat takes a fair amount of effort to set up and can be done with regular D20s almost as easily.
It is in general better to use systems that make cheating harder. However, regular D20s are less accessible for most Magic players, and I'm not sure if forcing everyone to go and buy non-spindown dice is worth it just to offset such low-value potential cheats. If you're really concerned about cheating you shouldn't be using dice at all; use something like this instead. (Most phone apps like MTG Familiar also have a dice roller.) I'd encourage people to default to non-spindowns when they have one on hand, but personally I wouldn't have any problem with someone rolling a spindown die in my games for now.
Do note that in sanctioned tournaments, spindown dice are prohibited by the Magic Tournament Rules. With regards to casual play, I'd like to to quote an 8 year old Reddit post:
Friends, do everything in your power to avoid playing with anyone that has a passionate opinion about whether or not to roll a spindown die in your fantasy card game. Your life will be so much more enjoyable if you don't seek out conflict over something this trivial.
