Going by match stats, constant winning.
A faction winning/being strong doesn't necessarily have anything to do with how boring/frustrating they are to play against.
UKF is dogshit, but still very boring to play against.
What makes UKF more boring to playing against than OKW?
Extremely static and defensive play that's very difficult to break for weaker players/players without certain tools, but becomes trivial for higher level players/those with access to certain tools. A primary example being the Mortar Pit. It isnt fun to play against emplacements.
As well as particular units, such as the oft overtuned nature of Sections, which often necessitates opponents go for very specific strategies, and the UC, which can be exceedingly frustrating to deal with due to it's fantastic bleeding power, and subsequent upgrades that make it very very potent.
OKW, of course, have the Kubel, but that's much more fragile, and doesn't scale nearly so well.
The latter two are functions of UKF lacking some fundamental tools, and the design team being hamstrung by Lelic so they can't do away with some abysmal design decisions (Bolster, etc), nor can they patch the gaping holes in UKF's roster, both of which would allow the remaining parts of the faction to be made less frustrating/boring to face.