Thanks for the responses, guys!
Am very interested in all of this move through cover this army seems to get, means almost nothing will slow them, but without any major transports a "fast" moving army may be harder to build.
Unfortunately, the Ruststalkers are not one of the units that gets Move Through Cover. They do add 3" to all Movement, Run and Charge moves, though, which I think results somewhat similarly. They do move quickly, but the lack of transports does hurt. Hopefully the rumors are true that we will be getting one this summer!
I'm intrigued by the Ruststalkers killing a squad of Assault Terminators; what always turned me off on Ruststalkers is that with Str5/6 depending on loadout and AP5 (or AP-, whatever Transonic weapons are on the first round), they're not very good at killing their supposed ideal targets until the second round of combat. It's a super cool bit of fluff and a fresh mechanic, but unfortunately their ideal targets are usually pretty deadly with Str6+ attacks; how did you find they fared against that kind of thing? To me they seemed like they'd be good at killing Tacticals or Immortals or a Tervigon; tough supporting/ranged units that don't have the melee punch needed to break through W2 FnP. Their mobility with Dunestrider helps in that regard, easily able to reach vulnerable support units that are hiding just behind the front line/away from the main bulk of the fighting. I'm pleased to hear they can do work against dedicated melee units too though!
I think my encounter with them was largely luck. The more I look at their stats, the more it seems like I should have been hurt just as badly as I dished it out. The Terminators had Lightning Claws (it was only a 750 point game, so Thunder Hammers and Storm Shields would have been extremely pricey), so no Instant Death, but I think my opponent forgot they were Armor Piercing 3 and I'm unsure if he ever used his rerolls for Shred. I would be more hesitant in the future to charge Terminators again (unless maybe I had Infiltrators nearby to let me hit first and hope to kill a lot of them before they hit me back). I also had used the huge Weapon Skill doctrina that turn, which likely reduced my received wounds considerably.
Next time I play this guy, I will probably try to shoot the Terminators more (I could have gotten the Ruststalkers out of there and tried kiting them around with Vanguard, who might drop some from sheer numbers of wounds). He likes using a Chaplain in Armor Indomitus as his Warlord, so something like that would make a much juicier target for them. I would not be at all surprised if future encounters like this result in mutual extermination or Terminator victories, depending on how we roll.