My computer that was good enough for sc2 is broken atm, so I havent played for like 3 months of so. Although when I was playing marine arena quickly became my favorite map and I can beat pretty much anyone with a low level merc and some really upgraded marines. If I want an easy game I'll jst upgrade marines and get those war of the worlds alien looking merc protoss units, forgot the name. Also, I can beat pretty much everyone in nexus wars, but that gets old for me and I used to play it while semi afk.
I played one risk on sc2 once and I was sorta dissappointed. The battles, like you said, were just too quick and even when similar appearing giant balls of units were moving at each other to fight (sc2 armies for some reason like to group up in a giant mega ball it seems) one army would totally dominate the other one in like 20 seconds.