umm, there was only one for sure CG explosion, the rest being some pretty cool practical ones. And I wouldn't consider the trailer much CGI, this trailer showed quite a bit less than say something for Marvel or any of the Transformers movies. Dear god, the amount of iffy CG in Age Of Ultron really took me out of a movie, that was already so-so to begin with.
I believe they have a perfect balance between the two at least in the trailer, plus I'm sure all those shots will look better later.
And just saying ROTJ's final space battle was pretty crazy, especially for the movies of that day.
I do understand your worry though, as I always found the JJ trek space battle scenes really iffy quality, so I hope they actually hold up here. The enterprise tended to look quite fake many times in those films, and I hope that isn't the case with stuff here too with the falcon and such. And hoping the x-wings' quality improves decently as well.