That's implying Darius is able to snowball in the first place; with the current meta being all about the tankiest of champions when it comes to top lanes, the only way Darius is gonna show some zoning potential without grabbing a few kills is by rushing brutalizer and potentially ruining any defense you might have back against the enemy. Brutalizer isn't always the most effective thing even so, since Giant's Belt is more of a thing than building armour is on top champs. Whether Darius can make use of zoning the fuck out of his enemy or not depends entirely on whether he can grab a few kills early game or not, more so than other top laners, because when it comes to duels in the top lane, the enemy is most likely going to be tanky enough to finish him off before he can secure a kill with his ult, meaning he'll have to use it early to get the extra burst in before he gets his ass wrecked, meaning that whether or not the reset is permanent or not doesn't even matter anymore.
Darius is gonna have to steamroll decently in order to actually have the ult to zone the opponent, in which case you're absolutely correct regarding the matter. But if Darius is just doing average or worse his ult won't really have the same power of scaring away the enemy because they'll be tanky enough to combat him and take him out before they're low enough to get ulted, in which case he'll use it for burst early if he can finish them off; in that case, there won't be an immediate zoning effect and he won't be as terrifying for a bit. Since this is the case, he'll most likely have it in use later on when he's now in top form; or so he would be if his ult had a permanent reset; but because it doesn't, it'll be pretty easy to run out of any core power when it comes to anything which isn't a quick teamfight. Darius is risky in that whether he does good or not depends entirely on how much shit he's managed to wreck, even more so than other top laners.