If your GM or DM doesn’t allow spindown dice in their tabletop roleplaying game, then that’s the rule, and you should stick to it without protest.
However, it may be the case that the standard d20 they would rather you use isn’t much better than a spindown dice.
Spindown dice aren’t designed for roleplaying. Instead, they’re intended to be used as counters in games like Magic the Gathering. The numbers 20 to 1 are in a sequential pattern on the dice, with 19 next to 20, 18 next to 19 and so forth. This is in contrast to a standard d20 on which the low numbers are spread out equally among the high numbers.
In this video, the Four Sided Guy explains the differences between d20 and spindown. The different placement of the numbers is designed to help compensate for imperfections in the dice. Perfect d20s are rare. It’s commonly the case that the dice are put in a tumbler and this can create subtle changes in their shape. It’s also possible that spindown dice made with cheaper materials, more quickly, as they technically don’t need to be balanced to serve their purpose.
In the test above there turned out to be little difference between one d20 and one spindown. It wasn’t a huge test, though, just 100 rolls and only on one dice.
An unbalanced spindown, should you happen to have one, is more likely to favour a set of numbers (high or low) than an unbalanced but otherwise standard d20.
Standard d20s, though, could be better.
Henry Segerman of Math Art Fun runs a Dice Shop which sells numerically balanced d20s.
These even better-balanced d20s pay attention to their number clustering so that air bubbles below the surface are less likely to favour a set of numbers.
If you total the numbers on the vertex of a standard d20 then you’ll get these totals: 39, 47, 49, 51, 52, 52, 53, 53, 54, 56, 58, and 66. In other words, a d20 with an air bubble that favours the 66 vertex is likely to roll much better than a d20 with an air bubble that favours the 39 collection.
The Sergerman balanced dice do better. The vertex totals of their sides add up to the following; 52, 52, 52, 52, 52, 52, 53, 53, 53, 53, 53, and 53.
The Dice Shop domain seems to be down, but the numerically balanced dice and other interesting variants are still available from Math Art Fun’s shop.