Plus side of the 21 is that it won't require a rework of any existing magazines especially rotary.
Downside would be oddball bore size, so available bore blanks and off the shelf bullets available to other companies will be limited. While being Sammi it's fair game, cost to entry will be barrel tooling for gun co's and whole new bullet diameter for an ammo maker.
Both issues are solvable but the greater question is "why". Looking at the cci/speer/winchester/federal near hegemony, they produce the lions share of domestic non toxic 17 hmr, hm2, and 22 wmr. Why on earth would they want to adopt new tooling for a competitiors new mousetrap. Going to take a tremendous amount of sales to get enough fomo for them to wade in.
Have a childhood friend with a 19 calhoon.... it's not exactly a plethora of barrel mfgs lining up to make barrels for it. Heck half the companies don't even bother with 204 or 17 as the sales are to small enough volume. I'm sure someone will tool up, but not likely a large number. Trickle down effect will mean even less boutique companies are less likely to source hard to find blanks.
Seems like had they just added a little shoulder ala 17 hm2 and worked on the pressure spike issue they could have gone .204 diameter and solved most the negatives. At any rate it will get bought, tried, and likely relegated to the dustbin of the 17 Aguilla.