R
Yes, https://www.cyberpunk.net/en/news/37801/patch-1-2-list-of-changes :
Cyberpsychos and minibosses are now immune to Tranquilizer rounds and System Reset Quickhack.
That's because System Reset trivialized those fights that were meant to be challenging, as it could instantly defeat any enemy non-lethally.
While it's possible that later updates added more immunities, in my experience (before update 1.2), there were no enemies that were immune to any Quickhacks.
As for why Reflexes is recommended for a Quickhack build, my guess is:
Critical Error allows your Quickhacks to benefit from Critical Chance and Critical Damage. Critical Chance is affected by Reflexes, and even without Critical Damage, your Quickhack will deal 50% additional damage per Critical Hit, making Reflexes (for Critical Chance) more valuable than Cool (for Critical Damage).
Generally speaking, Quickhacks synergize better with guns (Reflexes) than stealth (Cool). Using Quickhacks may interfere with stealth by alerting nearby enemies (some Quickhacks don't), but it won't interfere with guns.
Technical Ability is mostly useful for its ability to craft legendary items, some of which are incredibly powerful, but do not directly affect a Quickhack build.
Body doesn't really synergize with Quickhacks, but it doesn't interfere with it either. Having more health and stamina is never wrong, but Body favors melee, while Quickhacks are closer to ranged attacks. Guns would allow you to fight back while waiting for your RAM to recover, without forcing you to move from your current position.
This means that Reflexes is simply the best attribute to invest in, once you've maxed out Intelligence.