I'd put it this way: HFTs are better than nothing but they are not acoustical treatments that help with things like early reflections and the like. They help reduce high frequency energy.
How do I know this? I'm living it.
I have a large open floor plan that is a challenge from a logistical and aesthetic perspective. (Okay, WAF and related.)
For this reason I've resisted acoustic treatments and when I found out about HFTs, I thought they seemed like a great alternative
To date, I've deployed the HFTs, the Wide Angle, Speaker Kits and one Black Box. All nice stuff. It all helps. I would characterize the effect as quieting the room. I notice more clarity and space.
But, it's not the same as treating a room acoustically. I had a professional come over and do a room analysis. I already knew this but wanted to prove it with data. It's clear. I have to treat my room/space to the degree I can get away with. That means some permanent treatments and some that can be moved in/out of the room.
Again, I really like HFTs, etc. They did help. And they are very unobtrusive. But, for me, they're not enough.
I'll report back once I get my treatments in place, probably sometime mid-summer.
Hope this helps.