No, it isn't just you. What you experienced are complaints by a lot of users. It is one of the reasons that users of the packs are seemingly polarized about loving or not liking them. As an equipment tester for their packs, I had a similar experience. As experienced as I am at pack fitting and adjustments, I could never get it to "ride" correctly. One thing or another would feel "off" and despite my best efforts could never quite dial it in.
Now, I could definitely backpack and bushwhack with it (although bushwhacking is not kind to cuben fiber or thinner nylon bags), but it would be a "forward" presence; it would alway let me know it's there. A pack that fits and adjusts properly will not make the weight in a pack disappear, but you generally forget about the pack itself.
The great thing about Zpacks is their customer service and the general quality of their equipment. I own their poncho, their tent, and their rain jacket. I love them. But the Arc Pack Series and I don't get along
BTW. the CleverHiker site gives the ArcFlex a big thumbs up, while acknowledging some weaknesses
And I agree, for those that are able to get a good fit, it is a good pack, and will certainly help reduce the 'back sweats'.