I think for the average exerciser just getting out there, yes they are great.
However, Ive used fitbits and apple watches and I always go bak to my trusty Polar.
I have lifecycle app on my phone that tells me my steps and a whole lot of other information. Since I always have my phone on me it's quite nice and also having SleepCycle app the track my sleep as well.
As for real fitness, the fitness tracker devices were very unreliable.
I would wear my polar FT4 and the fitness tracker would be so off.
It was annoying. I would like to get the bluetooth polar to connect to the app and to get a bit more details of my workouts.
I even remember a HUGE long post about how unreliable Fitbits were when working out.