Well, for 4E at least, there's no real mystery to it, IME, the "Rule of 3s" is generally a good guide. And I don't see why this would be any different really for 5th, until perhaps you get into the elevated Skill echelons (as armor generally doesn't have quite the up-scaling capabilities as skills). There's Limits to consider for 5, but that might play out as a "Rule of 1s"...?
Putting your PCs up against an equivalent number of NPCs (with equivalent capabilities, i.e. w/ a decker and magician) with relevant dice pools (attacking, defending, soaking) of ~3 fewer than your PCs, is probably going to result in them winning but having to expend some resources (generally in the form of recuperating damage/drain, etc.) at the end of it.
NPCs of ~6 fewer dice is going to result in them winning handily, probably not even having to expend much for resources.
9 fewer and so on down just gets exponentially more and more dis-balanced.
The caveat to the lower dice pools being that you probably want to keep initiatives (and damage soak) of the NPCs as competitive as is reasonable, to the PCs. Initiative is huge in balancing combat, but I would think not as polarizing as it was in 4E with the dropping of IPs. I would also say that you could add +50% NPCs, per -3 dice pool disparity to the NPC party to bring it up to an equivalent challenge. So, a -3 NPC group with 1.5 x the number of members as the PCs, should pan out in a more fair fight. 2x the number of PCs for -6 NPCs should be more fair (PCs should end up expending some resources), though this may be more logarithmic as when you're approaching -6 and weaker, your mook's shots are just gonna be bouncing off, more likely than not... Unequal capabilities can be very dis-balancing with this rule, especially when it comes to magic. If the capabilities are unequal, but you want the fight to be more equal, just add +1 or 2 mooks per lacking capability.
NPCs with approximately equivalent dice pools could go either way (may have a player or two go unconscious), depending on how rolls pan out, but as long as the PCs have even a slight edge in initiative and damage soak dice, they're gonna win (generally).
Equivalent numbers of NPCs with ~3 more dice than the PCs should present a serious challenge, requiring good tactics and smart use of resources to win (and you still may end up with someone unconcious - hopefully not dead, but it's a possibility).
6 more and so on up will be harder and harder, I've never put my PCs up against NPCs higher than +3, as I'm pretty sure they'd die.
Ya dig?
So you have to know your group pretty well, but if you have several "cookie-cutter" NPC templates (from joe-schmoes, to elite ops, and prime runners), this is a real easy way to make generally balanced combat encounters even on the fly. This is the "formula" I've used from the start in 4E and it's worked really really well. Receiving much positive feedback from my players (which includes past SR GMs) in my ability to balance encounters for Shadowrun.
{EDIT: SORRY I EDITED THIS A TON AS OTHER THOUGHTS CAME, IF YOU READ THIS BEFORE THE "LAST EDIT" TIME, MAY WANT TO RE-READ}