I’d say the problem id primarily the range and angle of LoS they need to cast, and even then, the range is secondary; they can, of course be tracked through their casting and most Careers have options to deal with thing even that far away.
But by the “angle” I meant that they only need a momentary LoS, and it can be so narrow that no human player could use the same to do anything. This has caused problems before, particularly in Skittergate’s Gatekeeper arena, where visibility (and firing angles) to outside is very limited, but Blightstormers could still use those angles momentarily to cast their storms while we could do nothing to retaliate. They also tend to stay at at least moderate range after casting, i. e. outside the arena. It seems that we’ve had less troubles out of them lately, but I don’t know if that’s just luck or if something was changed at some point. Range-limiting their casting wouldn’t often do nothing, as only a few maps have visible ranges long enough to be worrisome (Athel Yenlui and parts of Against the Grain and Skittergate come to mind), but I wouldn’t say no to it.
As for the storm dissipating if LoS is broken, I think that would make things too easy. The sorcerers teleport away as soon as they take any hit, and are unlikely to 'port to visible locations, so any damage at all to the sorcerer would be likely to break the spell. They also wander around slowly while the storm rages, and are quite likely to break LoS themselves during that time. If some ease would be wanted to that, a realistic (and more limited) approach would be that the storm can only target players and follow them as long as the sorcerer has LoS. I don’t know how well that would fit the lore, i. e. whether the storm has a mind of its own (like it seems to now) or whether it’s the sorcerer that keeps controlling its direction, but it would give us a better chance at avoiding the storms.