I have not flown WN in a long time..but I have booked a lot of tickets there recently for an A list friend of mine who is also a UA 1K..she does a lot of flights intra-California out of SMF in addition to her UA travel and WN works better in those cases.

WN does NOT have the lowest fares.. in fact their fares are often more than UA's for comparable flights. They compete.

While their website might have seemed nice, friendly, and easy for the simple round trip the OP mentioned...Their booking engine is horrible and cumbersome if you need to veer from the simple round trip even slightly, or when you start to frequently book trips with family members and have to re-enter their information every single time.

I have read that they are implementing a major upgrade soon that will bring them up to speed with other airlines. Not being able to book an open jaw or multi segment trip online without calling makes them a dinosaur in 2016. If you used it awhile you'd see there are tons of functions missing... booking travel for companions (saving traveler information for frequently booked companions under your profile, etc).

Having no assigned seats means they can board in the way they do. I'm fine with UA's boarding... but from a group 1 perspective.. I can see there's room for improvement..