Is an Explorer that beats the hell out of other explorers an excellent unit?
2 free scientists come way too late and do not contribute that much to space race victory, you should try it, in fact by the time I usually build the UB I already have half the spaceship, so yeah great UB...
Considering how many mounted units get fielded by other civs from the knights era onwards i'd say a unit that can easily counter a large part of your rivals army is very powerful indeed
If the explorer was a key military unit of it's time an got built in large numbers yeah another explorer that owned it would be very good indeed