Yeah, the volume might be relevant. But the primary opportunity for water to enter is presumably via the reservoir cap, and if you leave that on, then water shouldn't be getting in. If water is getting in, then apparently it's entering at an extremely slow rate, or 18 months (22,500 miles) wouldn't be nearly often enough.
On the Niro, as with many other cars that have a hydraulic clutch, you use DOT-3 brake fluid for both the brakes and the clutch. So the same concerns about brake fluid (particularly the concern about it attracting moisture) apply equally to both.
My Honda dealer recently recommended that I change the brake fluid on (what is now my wife's) 2008 CRV. It was kind of discolored, the car's in really good shape, and now that it's my wife's primary car, I'm a little more inclined to perform certain kinds of preventative care than I would be if I was driving it (partly because I love my wife, partly because I don't trust her quite as much as I trust myself to notice when the car has a new quirk that might need attention). In the past, I've only changed brake fluid or clutch fluid when I had the lines open for some other reason (broken flexible brake line, leaking slave cylinder, etc). It was a brand new experience to me to change the brake fluid as just a preventative kind of thing, but I decided it was worth doing (except, not at the Honda garage). I checked just now and I see that Honda didn't include changing the brake fluid in the scheduled maintenance table for that car, but they did put in a foot note recommending that it should be replaced every three years.
My bad, I've gone for a really long time with a number of cars where I never changed out the brake fluid. And then on a couple of them, I wound up doing it because I had to bleed the lines anyway due to some other maintenance that was needed. I don't think changing the fluid would have prevented flexible brake hoses from springing a leak on cars I owned in the distant past, but it might have prevented a clutch slave cylinder from leaking, maybe.
Although the Niro Owner's manual is pretty clear that the clutch fluid should be changed every 18 months or 22,500 miles, it only requires that the brake fluid be inspected, doesn't require it to ever be changed (although I suppose a dealer might inspect it and then recommend that it be changed).
Given that several people have stated that their dealer didn't know how to perform this service, while others said they paid upwards of $80 for it, I might wind up being one of those people that says the dealer isn't touching my car (for this particular service). Assuming you can get away with doing it without needing to retrain the computer and with just the investment in a power bleeder and a bottle of brake fluid, I might wind up doing this one myself when I reach the point where the manual recommends that it should be done. My total cost would be slightly less, I'd have confidence that it was done thoroughly (rather than wondering if maybe the mechanic just changed the fluid in the reservoir).
I'm a fan of having the dealer change my oil because that's usually a "loss leader" for them and the few dollars I save in doing it myself doesn't justify the effort, time, mess, and also because they record the event and also because they are likely to tell me about things that I might not find out about if I didn't visit them. But when the service cost goes up and the service manager is signaling that they don't normally do that and don't exactly know how to do it, I might rather be the one that learns how to do it right, rather than paying the dealer's mechanic to learn at my expense and with less interest than I have in ensuring that it's done right.
2018 Niro PHEV Gravity Blue