In general, the smaller your plane, the more likely for delay or bus gates. The Q400s, esp. the Portland shuttle are most frequently cancelled due to low ceilings / ATC - fewer passengers impacted and with Portland, almost always another flight later. The E175s also get the short end of the stick in ATC delay to Alaska scheduling decisions. I also found that later at night the gates are typically already filled with planes staged for the morning flights so the late inbound flights go to the ramp by the cargo area and you get bussed in.
While that's true, in my case it was a 738 from RDU that got hit with the ATC delay and the bus gate and the E75 to BOI that left early from a normal gate.
