I think the mirror universe arc was the best. The idea of going into the far future was good, if poorly executed. I also worry they’ve written themselves into a corner now. It’s going to restrict any media set post TNG but pre Discovery.
It’s no more limiting to TNG era stories than the TNG era itself was to TOS era stories. They can’t blow up the Earth or genocide any major races, but beyond that we’ve been given very little information about any character’s future. I didn’t find Star Trek VI any less exciting because I knew the Klingon empire would still be around 80 years later, and I’d say SNW is flourishing under far tighter restrictions.