A war machine is completely plausible. The only problem is that, like just about everything else, it will have a weakness.
Examples in the Starcraft universe include:
Terran Battlecruiser and Protoss Carrier: Vulnerable to Scourges. The latter is also vulnerable to EMPs.
Protoss Archon: ESPECIALLY vulnerable to an EMP.
Protoss Dark Archon: Its most powerful attack (well, it's not really an attack, but a spell) also drains its shields. Like its light counterpart, it's also susceptible to an EMP.
Zerg Ultralisk: Powerless against air attacks.
Terran Siege Tank: High cooldown, can't move in Siege Mode, vulnerable to swarms.
Protoss Reaver: Slow and has low HP.
Zerg Guardian: Relatively low HP, vulnerable to air.
Zerg Devourer: EXTREMELY high cooldown, vulnerable to ground attacks.
There's also some other stuff in sci-fi as well, like in Star Wars:
AT-AT: Has vulnerable limbs. Also vulnerable to attacks from below.
Death Star: The exposed thermal exhaust port. 'Nuff said.
Star Destroyer/Super Star Destroyer: The bridge is relatively exposed (watch the scene in Return of the Jedi where a wayward A-wing takes down a Super Star Destroyer by crashing into its bridge).
Trade Federation Droid Control Ship: Sparse defenses and low shielding in critical areas (like the reactor).
AT-TE: Has several weak spots where a missile could easily penetrate and blow the whole thing up.