Well, that's not exactly true. If the PCs are ALL using something, there's a good chance that most other special ops agents are using it too. But it doesn't necessarily mean it's the right choice for rent-a-cops or corpexec bodyguards or Johnsons or Stuff Shack cashiers or anybody else.
(And for that matter, it doesn't even mean that all 'runners favor it. People aren't always rational, especially when it comes to gear.)
True, and I should have expanded on that thought a bit. Here, is a less concise version. Keep in mind that this applies to any unusual piece of equipment. This could as easily be about Form Fitting Body armor or Pepper Punch grenades.
Here are my basic assumptions about gameplay that underlie my point:
1. People are generally not stupid about self-preservation or their job (loosely defined as whatever they do for livelihood).
2. NPC's are going to favor the most efficient gear just as much as PC's, subject to their budget and their goals.
3. Shadowrunners are going to face tougher enemies as they advance, rather than the same ones over and over.
4. The PC's are generally a fairly good microcosm of the world they live in. If all of the PC's are using something, its a good indication that it is useful to Shadowrunners and often to the people who try to stop Shadowrunners.
5. Any character with applicable skills knows more about that aspect of the game world than the player does. In other words, anyone with a firearms skill of some sort, anyone with a combat Knowledge skill or anyone who reads "Solo of Fortune" knows that SNS rounds cut right through armor, taze and often KO targets very efficiently. If your PC's know it, so does everyone else.
Based on those, go-gangers probably won't use SNS. For them, looking manly, intimidating and drawing blood is more important than pure stopping power. On the other hand, PC's will have probably quickly advanced beyond being challenged by random gang violence. Rent-a-cops, bodyguards etc. are concerned first and foremost with stopping power. Budget permitting, they're going to use SNS on DV<6 weapons and probably APDS on DV>7 (your math may vary). Whether the person lives or dies isn't going to matter that much in their ammo buying calculus. Fewest shots to incapacitate is. So even as the PC's are adopting those SNS rounds, the gangers aren't. However, those same gangers become less of a threat every time karma is handed out.
On the other hand, the next corpsec team may well have discovered the wonders of SNS. A dead 'runner is all fine and good, but a live KO'd runner can be interrogated or given a new mission in exchange for amnesty.
The first time any team (gang, corpsec, Shadowrunners etc) gets laid low by non-standard weapons, they're going to put some serious thought into plugging that weakness. If the PC's get gassed, they'll have gas masks next time. If Ares loses a prototype because of SNS rounds, they'll have protected armor next time. Note that an armor jacket with rank 6 electrical protection has an effective Impact of 9 vs SNS and 6 vs everything else. RAW you could wear non-conductive form-fitting armor underneath for another 7 against SNS (since form-fitting explicitly stacks).
Once that kind of protection becomes common (if not completely standard), the offense is going to come up with new tactics. When they do, their victims will catch on and both train on the same tactic and on how to defeat it. In a realistic world, Shadowrunners would almost always be on the losing end of this. Corps have a much higher budget for R&D and training. Runners might get away with a particular tactic once, but the corps can always afford to counter it. It might even be amusing to have a "run" where the runners get called in to consult unofficially with a corporate team about their tactics and how to use them.
The end result is basically the same as any house rule. SNS becomes a way to turn a regular pistol into a tazer. Its great for self defense against low-lifes, not so great against anyone who's planning for a fight against professionals.