I haven't written about it. My acting classes happened a long time ago. These days I'm more of a master of puppets.
Your impro mentioning brought back memories of watching this man pretending to be a schizophrenic, yelling, and rolling on the floor.
I was hyperventilating because I realized I was next and there was no way in the world I could act like that.
I got my turn and, while it felt impossible at first to keep up with him, I realized the show was on me, and I didn't really have to keep up with him. I got to set a different tone.
To continue your party of thoughts, both can be excellent experiences, but in different ways.
Impro acting plays the spontaneity card and forces you to be more resourceful. It can act as medicine for the overthinker.
Translated to writing, it lets you think things through and refine, tinker with words until you get something you didn't know you were capable of. It tames impulsivity a bit.
Also, impro acting builds on many layers. It's not just words, it's words plus volume, intonation, gestures, facial expressions. And while it may feel overwhelming, I'm sure that, in backstage, the brain is making connections you wouldn't reach otherwise (or at least not when you're taking advantage of the time lag specific to impro writing).
Translated to writing, you're stripping off those layers. It's just you with the words in front of your eyes (there's a reason they say 70-90% of communication is actually nonverbal.) So, you're giving those words meaning and direction based on very few clues.
Would it sometimes be "totally impossible to reply in total impro mode right away?". I doubt it's impossible. I believe you're saying you won't be happy with the outcome, the writing won't match your standards or expectations.
Here's a parallel between impro writing in total impro mode and... face-to-face conversations with strangers (it would be very different with people you know and feel comfortable with). Let's call them impro... wording?
Anyone who ever randomly talked to a total stranger got a chance to experiment with impro wording. Did it turn out a fascinating conversation? We can say they managed to word "some good stuff together". Was it completely awkward and strangled in silences? Not so good stuff, but still, an experience.
Impro whatever-gives-me-time-to-reflect is where I feel most comfortable, personally. Yet, I got to experience impro acting, and impro wording, and a few other impros that even though felt "totally impossible for me" at that moment, have taught me that I can do more than I think I can.
Too late to keep this short but, the bottom line:
Can't say one is better than the other. They're just different, and depending on which one is making the "actors" feel more comfortable, it's probably a good idea to try the other version.