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he terrans,

As of late I have started to love the viking more and more. Especially in PvT. The most important thing I rarely see is using them for harass. like, 6, landed vikings melt workers like nobody's business and are actually pretty decent against buildings.
I think it could be more effecient then drops because it takes less time to deploy them, less time to get them out again, and they are way better against storm, forcing a Toss to keep either stalkers in his base or mass cannons.

Maby this could also be a semi usefull thing in TvZ aslong as you have a good defence set up.

The problem I can see is that you need to have vikings with your army to prevent getting overrun by deathballs but I was just wondering if I'm missing anything or this is something I could actually go for instead of sometimes doing it impulsivly like I do atm when I have vikings I'm not really using.

Protoss can just warp a few stalkers to defend while he facerrolls you with his main army. Doesn't sound too good.

Ye i love vikings to and i think their actually pretty good on ground to , against zerg.
En mass it almost counters the ling bling muta strategy by itself , only needs support from a few hellions and a thor or 2. I think of them like a marauder that kills lings when on ground. Thought one must be pumping much more vikings than it is mutas if their gonna own the groundforce afterwards and you realy should have 1 more upgrade than the mutas to offset that it isnt a hardcounter your building.

Against protoss it's only semi usefull when he does not have blinkstalkers and his main army is in no position to be a threat to you for a long time.

Vikings are expensive and die really fast to stalkers, especially when they need to go into airmode before they can fly away.

Also as Pleurodesed mentioned, you made those vikings to counter the collussi, not having them with your army is dangerous as you will die without them. They are also quite slow to retreat back to you army should you need them.

Drops are better, they are less risky money wise and do not take away a critical part of your army should an engagement happen while you are doing the drop.