They can be very boring to play against and stall out the game for a loooong time.
Only in standard format.
Modern mill is crazy and every miller should be running Surgical Extraction by the fours. Don't even need the black mana, just pay the two life for it to get rid of whatever anti-mill they may have going since I known some people love to run Emarkuls, Elixirs, and other legendary Eldrazi vs mill in modern and the people who do run Surgical Extraction main board it. Plus there are creatures that trigger abilities of cards being mill of player taking damage from them, think it was one of the Guildmages that did that, but not sure which. I know it was some creature in the past m12-m13 set. Plus black mill is crazy with the one(can't remember card name) thing that makes player keep milling till they hit four land and then they stop, which is slap in face for opponent since they can be lucky or unlucky, either way that is four land gone. I known people who ran ghost quarters and snapcasters for side just to repeatedly Surgical Extraction non basic lands. Also if you go U/B, people run playset of extirpate as well(or the rich ones will, since snaps, surgicals, and extirpates will be emptying your wallet.)
There are effective ways to kill with mill or outright mill someone out, but standard is more time consuming tasks. Unless you running some Jaces(think he was reprinted, the mill twenty one?) or something like that.
But, I outran standard mill with Jeskai aggro beatdown, so dunno. In end it depends on draws, luck, and how you play in standard, just like any other format. I never lost to mill given I play low cost but heavy hitting creatures or will outright swarm opponent before they have chance, but only in standard. I lost to modern mill with same deck, just because it wasn't 'fast' enough.
Edit: Well, maybe just snapcasters will be emptying wallet. I could of sworn extractions and extirpates were double digits as well unless they had hit a price drop since a year or so ago of checking.