And then I'm struck with the problem of measuring challenges: how??? There aren't levels or anything to judge easily who should be up against what.
The rule of 3s is a decent starting point. Because a 5 or 6 on a d.6 is a "hit", 3 dice more or less is 1 hit more or less, on average.
So, take a look at your PCs and there main dice pools. Dropping or raising an NPCs similar or resisting dice pool by groups of 3 will give you a decent guide in "challenge levels".
Example: Say your PCs have Firearm Skill Pools (or Blades or Hacking or what have you) around 15 and Damage Soak Pools around 12.
Enemies with around 15 in Attacks and 12 in their Defense, if presented in equal numbers to the PCs, should be a hard fight, you may even put a PC (or 2) down.
Enemies with around 12 in Attacks and 9 in their Defense, if presented in equal numbers to the PCs, should be a challenge, but the PCs should survive. i.e. the PCs should be having to expend some resources to get through the battle or in the aftermath.
Enemies with 9s in Attacks and 6 in their Armor, if presented in equal numbers, should be an easy encounter. They may take some damage, but nothing major.
Enemies with 6s in Attacks and the same or less in Defense should be completely walked over by the PCs.
The nature of SR combat (the application of wound modifiers) is such that initiative is VERY important. Going first is gives you the upper hand. Why? Because he who goes first has the best chance to impose some negative modifiers on his enemy before they go and therefore his enemy, who may start out as his equal, will already be at a detriment. This makes Initiative the best place to tweak challenges, have weaker enemies spend their Group Edge to go first so they're not all just one-and-done. Give your tougher enemies lower initiatives if you're afraid they might otherwise kill your PCs. Make sense?
Armor is also a good place to tune challenges. Beef up enemy armor to at or above PC levels, but keep their Attack Skills -3 or 6 for enemies that give the appearance of toughness but probably won't end up doing that much.
And if you want to injure, maim or kill your PCs, give the enemies high enough Attack Pools so that they can take some damage and still be throwing a decent number of dice.
All of that said, combat is an order of magnitude more lethal in SR. One ill-timed glitch, or failed Damage Soak test can make it a really bad day really quick.