I've heard conflicting reports. Necrons have hax weaponry, as do Dark Elder, but no one except possibly the Tau and the Imperium Core Worlds are really exploiting planets to their fullest, Bolters are useful but not overwhelmingly more so than (movie only, make-walls-explode) Star Wars blasters/Covenant Needlers, and no one really makes the cannon fodder Imperial Guard, who I'm given to understand are Modern Army Men with the occasional strat nuke, obsolete.
Other Tier A armies: Partials, from the eponymous series by Dan Wells. Either side from Aldonoah Zero. Hidden Villages from Naruto. Schlock Mercenary, before the invention of the Teraport. (Afterwards, several factions jump two tiers with the Core Event and more recent developments, respectively.) So no, I suppose they're not totally underpowered, but they aren't top-tier impressive, even when they can focus force on a target. With their logistical difficulties, they'd probably lose to the others.