Basically what mass berserks said; but really, if you just look at stats your missing out on the unit. Mangudai have similar visible stats to a CA but are considerably better. In the same way, berserks are just plain better than vik champs. They have a crazy good patrol, due to their 18 attack, they regenerate very fast once researched, and they have considerably faster movement speed then champs. Berserks simply put, are designed for raiding.Bereserks are pointless to build because they cost 5 more gold and food than Champions, have to be built out of Castles (harder to mass), and have 12 less HP than Viking Champs.
All they get as a bonus is HP regen (very slow unless you get expensive unique tech), 1 more DAM, and 1 more melee armor.
Just use Champions.
If your opponent goes champs as maybe in a vik war or v aztec/jap, then you just mass 40 and run around to his eco and start wiping it out. They destroy tcs so fast and move so fast, that this would be almost unstoppable to infatry civs. I know this cause ive tested it out v carlos, and the only time it didnt work was wwhen my eco was so bad, I had after 5 mins running aroudn with 20 berserks like 10 reinforcements.
Against paladin civs, you stone wall, get relics before he masses his paladins,(hopefully at least 2) get all your casltes in one place prefeerably on a hill , and watch as you slowly win the gold war,
Berserks even do great raids v paladins, you can go in and destroy a tc and 20 vils before they reach you then just run up a hill, and lose 40 berserks to 20 paladins, which gold wise is a victory + all the vils u killed = totally cost efficient.
There is no way, champs can do this. Alsdo when both sides are totally out of gold, having just 2/3 relics can support occasional burst of 10 berserks which totally rule all trash easily, adn if you use them for side attacks you can vent the rest of your sstrenght in the middle and easily win.
Tarkans and berserks are very similar. Both designed to raid and not necessarily for pitch fighting. Although when micro begins getting tiring berserks can go on a hill patrol and wipe out 1 and a half their numbers before dying. While tarkans are jsut faster better raiders