It was really hard. In the previous country, we had a cozy, genuinely happy, settled life with lots of friends and community connections that we had built up, where the kids were building their own connections, where we were both comfortable career-wise and financially, and public services etc were really accessible.
We went from that to a struggle life in a (top 8 GDP) country where the people are much more closed off and many seem miserable and depressed, the school system is old-fashioned, and every aspect of daily life was/is much more complicated (tax, healthcare, buying a house etc). Coming from the relaxed comfy previous country, the kids were not happy, and 15 years later, only the youngest, who was 2 when we came, has any affiliation with the country and community. The other two have fled the country as soon as they could. And we earn 3-4 times the national average wage together and we can't understand how people who earn less manage. We also essentially came to help my H's parents, FIL was ill, but the dysfunctional IL family politics were so awful that we all stopped seeing them 4 years in. Most pertinently, I struggled to find my footing careerwise, and as the person who did all of the family's emotional labour, I often felt like I was drowning in stress, and I was constantly exhausted.
So the move turned out to be a real sacrifice for me personally and professionally and even physically (I was underweight for years and ground my molars to pieces). And while it was easier for my H (being his stamping ground), he felt the consequences of the move on me keenly. He knows now what "happy wife, happy life" means.
So yes, we regret the move: we messed up. It definitely wasn't perfect in the previous country - the people are friendly and relaxed but also shallow and complacent - and there are certainly good, even great, things where we are now. But it was EASY to live in the previous country, and we were able to build an EASY and comfortable life for ourselves and our kids. That ease and comfort shouldn't be taken for granted. We did, and regret it.
Also, if the burden and sacrifice of such a move particularly affect one partner (me in our case), this can create a very deep resentment, especially when the other partner is less affected because for them it's just slotting back into their culture and homeland.
You've already mentioned one very serious problem, which is your H's job. If this move means that he has to sacrifice his career, or the move will diminish his opportunities, that is almost certainly going to become a massive friction point for him - and your marriage - over the long run. No matter how well-intended he feels now, how much he loves you, and how willing he is to embrace the new life, if he is the one who suffers while you thrive, that will create an emotional ulcer in the marriage.
You also mentioned that he gets along well with your family and friends in Austria. I did with my ILs too before we moved to be close to them - because before we moved, we were VISITING them. Once we started living here, it was an entirely different thing. I would never have guessed what was lurking behind that lovely kindly facade of my ILs.
Anyway, this is my perspective. I was also older than you, I think (40s), the move was much bigger (transcontinental), and had 3 kids ranging from 10 to 2, so maybe that made things harder. But still, I tell anyone who is considering such a big life-changing move to not take for granted the easy and comfortable life that they already have, and to weigh that up against the possible - but not guaranteed - advantages of moving to the new country.