I finished the game, and as it turns out there are some enemies that are immune to quickhacks. Not many, but there are a few. In my case the two I remember are:
- NCPD officers if you do something illegal
- Maiko Maeda in the quest Pisces
The quickhack bar doesn't show up at all when targeting them. Although it's not ideal to not be able to quickhack these targets, it's also not a big deal - the police officers are completely avoidable and you can't really fight them anyway since they respawn indefinitely, while Maiko is not a high threat character and easily defeated (you may not even have to fight her depending on your choices).
That leaves the question of why bother with Reflexes. The reason is simply some enemies are not very vulnerable to quickhacks. Many trash foes take only 1 RAM to quickhack after Breach Protocol, but more difficult ones will take 4-6, possibly more, and it won't be easy to kill groups of those with quickhacks only. Boss foes are even worse since they have lots of HP to withstand quickhacks, and it can cost more than 6 RAM to quickhack them. You need another weapon in addition to quickhacks. What to use as the other weapon is up to you (Xbox Achievements' guide is right here), and you don't have to choose the ones tied to Reflexes. In fact, you don't even need to upgrade any of the perks at all, because all the weapons are usable - just less good - if you don't have points in those weapons. Finally, there are enough points to max Intelligence and two other attributes of your choice, and still have some points leftover (if you're a completionist, which I was).
It's true that Reflexes technically adds to your quickhack damage (since quickhacks can critical hit), but in my experience the quickhacks do enough damage to one-shot opponents even without a critical hit, so it's moot. There's also only one perk in all three Reflexes trees that I can see as useful (Deathbolt) and it's only a minor quality-of-life upgrade since health regeneration out of combat is cheap and you get health regeneration inside of combat from Body.
If I did another playthrough I would undoubtedly go with Body first to increase survivability and hence reduce the need to reload. Cool comes next, because 1) there are a few quests where it's important to stay stealthy, 2) quickhacks hit from stealth so it's easier to progress the stealth trees to their ultimates,* although admittedly the Cool ultimates pale in comparison to the very important Master RAM Liberator ultimate perk in the Intelligence tree, and 3) running faster is another nice quality-of-life improvement when moving between objectives.
Finally, quickhack builds are noticeably weak in the early-game when enemies don't die to a single quickhack and there isn't enough RAM to hack everyone anyway (the best Cyberdecks also need 40 Street Cred). Fortunately, perk reset is cheap (even though by the end of the game, perks are also cheap - I had more points than I knew what to do with). You can, and maybe should, start with something else and respec into quickhacks.
*The way my playthrough went, at the end of the game with every side job, gig, and NCPD alerts done, I still had only one tree (the quickhack one) that reached level 20.