If I had to pick the #1 thing that's keeping me from doing a site in Drupal 8 right now it would be that I haven't found a client willing to do a Drupal 8 project yet. I suspect I will soon.

But for certain kind of projects, I need more experience with Drupal 8's migrate system before I can recommend migrating sites into Drupal 8.

The one thing that's keeping me from Drupal 8 is the lack of support of my current hosting provider. My hosting server doesn't fulfill the requirements.

Am using Drupal 8 but if I were to name one reason not to, it would be the lack of contrib modules.
I actually wrote an article tackling the exact question you are asking: What to keep in mind when creating Drupal 8 projects - for developers

Learning a new syntax (Twig) for front-end development. I just don't have the time.

With a D6 site, using hundreds of modules, 1/2 of which are customized, we need at least a year of lead time to do an upgrade. It requires buy in, lots of money, and a full life cycle no less than one year in forward planning. Having a reliable life cycle is vital. If it isn't ready by xyz date, then it doesn't go in. You don't hold off launching indefinitely.

Because D8 is three years late, I have become Cassandra at my organization. After years of me saying, it's coming, they no longer believe me now that it has come. I don't know when and if I can get buy in. Maybe for D9?