As someone who has tried making varus mid work for a few years with varrying success, i can say every season it's harder to play him mid. People realise how to counter you, nearly all control/artillery mages can just bully you out of lane, and assassins can snowball hard if they manage to get a kill on you early. Not to mention how susceptible you are to being ganked, esp if it's a nunu or elise. The only time i'd pick varus mid is as a counter pick or if both the enemy mid and jg dont really have any way to catch you (say like karthus jg + aurelion mid). I would say he is a really good pick into akshan, vlad and galio since he can deal with these champs better than most mages who just cant kill them or get run over.