From the Mileage Plus Explorer Card brochure that came with the card - -
Any seat, any time with your miles.
. . . no limitations, no restrictions, no blackout dates. If seats are available for sale, they can be redeemed with miles at Standard Award levels.
Standard Award levels are 50,000 miles for a domestic round trip ticket, twice the Saver Award rate. I have the card but haven't used that feature. It would be handy for last minute emergency travel but otherwise I'd try to book earlier at the Saver Award level of 25,000 miles.
We've had a United Mileage Plus card for years and have enjoyed numerous free flights. I figure the "bags fly free" feature (1 bag per person, 2 people max) for UAL tickets purchased with the card will more than offset the higher ($95) fee for the card. I don't know if a FF miles ticket with the fees paid with the Explorer card qualifies for the bags fly free benefit.