One big downside is that Alaska Airlines has no premium economy (Economy+/Economy Comfort/Main Cabin Extra in UA/DL/AA respectively). Alaskan doesn't have these seats. You will get exit row seating on both Alaska and most partners, but there are fewer seats available than premium economy types.

Other downsides of Alaska are no mix-and-matching partners on one award. And there are not that many options to earn miles through CC bonuses/spending.

It is good if you fly a mix of AA/DL, and fly a lot to/from northwest US.

If you can reach mid-tier elite, UA or AA would probably be a bit more rewarding.