My complaint about J.J. is his writing (and who he picks to partner with 90% of the time), not his directing. The Star Trek films showed he's a perfect Star Wars director.
The key for me with my Rian comments is that he's writing both, not just VIII. Even if they somehow convinced J.J. to write all three, it'd likely be better than three separate writers, for example.
The directing doesn't bother me as much because there's a formula that I imagine 500 suits at Disney will not allow you to touch.
I think when you're starting (or effectively so) a single universe it helps to minimize the number of replaced writers or directors. Even when the writers are bad, they keep a consistency that helps build the universe/characters better than if you ping around to five separate decent writers.
First two Iron Mans had Jon Favreau directing (and RDJ "writing" all his scenes), first two Avengers had Joss Whedon writing and directing, James Gunn writes and directs the Guardians films. Markus and McFeely have written/are writing Captain America 1, Thor The Dark World, The Winter Soldier, Civil War, Infinity War. Russo Bros will direct Winter Soldier, Civil War and Infinity War.
Over in the DCFU Snyder, Goyer and Terrio are the ones who are doing/will have done Man of Steel, BvS, Justice League 1 and 2.
These will all connect thematically, even if like Ultron individual parts aren't as great it maintained the characters and everything "arc" wise from the first.
This was something that concerned me when they split the films up not just among directors but originally writers (IIRC, Rian was supposed to do a side-story not write both VIII and IX) especially letting J.J. go first and bail out completely. (I had always assumed he'd at least direct the trilogy to be honest.)
I'm pretty sure Colin Trevorrow got it because of Jurassic World's big dollahs, which he's writing the sequel (2018) of too which is why he won't write IX apparently. (Thankfully?)