I agree but you have a few flaws: Medusa can kill a shadowrath unless it uses shinobi the exact right time.
Full shadowrath army>Full spearton army because the shadowraths can kill the statue faster and shinobi through in case of shield bash!
Shadowrath have 2 less bar, and no armor.
Best used to kill magikill!
And also shadowrath would be the fourth (possibly third if it could shinobi the right time with good micro) at 1v1 combat with shinobi if you just pressed attack and used spells.
Well, yes, shadowrath are well-suited for assassination as well; but the main point is that in a no-micro match, a spearton without upgrades and a shadowrath without upgrades are about equal. However, this does not make the spearton as good as the shadowrath; If the spearton tries to run, it will get killed, but the shadowrath may run any time it wishes. With the upgrades, there is no way at all a spearton can kill a shadowrath.
The point is that if we are talking about troops as individual units, the shadowrath is undoubtedly the best unit of them all. It's fast as an upgraded crawler, it's fixate makes it possible to win seemingly stalemated battles, and its shinobi ability is immediately deadly to all ground units but giants.
How do you shinobi through shield bash, though? I tried once and got kited until I lost shinobi, whereupon I had to rush through speartons to the range of castle archers and lose the central tower as well as two shadowrath. Shadowrath with shinobi aren't as fast as speartons, so I usually send pairs of shadowrath on assasination missions until I kill enough of them or they get flying archers.
Anyway, I just want to say that speartons are quite possibly the weakest of the troops on their own; they are only used so much because archers and fliers could use mobile miner walls. In and of themselves, speartons are of little use to anyone.