I don't own one, but I have a friend who has nothing but good things to say about it. That being said, there is a lot of speculation that a future update might kill SKY3DS. People will say that the fact Nintendo hasn't blocked it yet is reason to believe they can't, which might be true, but the evidence suggests that it should be possible. This wouldn't be the first time Nintendo took an absurd amount of time to block or patch something.
Same as Gateway. 9.3 update killed Gateway support, so even if 9.6 (or a higher one) would kill the Sky3DS V2, wouldn't be a big deal. Especially as we know N3DS won't have 9.5+ emunand support for now, so the N3DS won't be up-to-date, even on a Gateway).
So to keep using it, you would just need to not update your sysnand, like you are already doing for the Gateway.
And if you get a Sky3DS waiting for the N3DS support on Gateway, it's again less a problem, as you won't update and stay on 9.0/9.2 (where the Sky3DS will always works).
I would say the big flaw of Sky3DS on N3DS is you can't play foreign games with the region-free exploit (you can use it with a Sky3DS on old 3DS/2DS).
Saves corruptions, dunno, but I also got one with the Gateway so I assume it can always happen (and I never really trusted saves on any flashcards, even older systems ones). When I care about the savedate of a game (Pokemon, for example), I buy the original game.
So for you answer, it do the job pretty fine for a N3DS. It play games of your N3DS region without any problems (though sometimes you have to wait a bit for them to update the template for the newest roms, like Majora's Mask where the update took a few days).
Of course, as you already know because you have a Gateway for your 3DS XL, a Gateway would be better but as there is not support for N3DS right now, it's either Sky3DS or nothing, so of course, Sky3DS is far better than nothing (not hard to do).
I would just advice to not put too much roms on it, or it could then be a pain to choose the game you want to play. Better just put 4-5 games at once, then to change when you don't want to play them anymore, as each game switch take ~8-10 secondes.
I personally bought one to use my N3DS while waiting on the Gateway release (seems to be what you also plan to do), as I also have a Gateway for my old 3DS.
I am aware that it is not perfect, which is why it should be possible to block it in a future update. This is bad for SKY3DS in particular because it requires updates to play newer games, play online, and access the eShop, but some people will claim that SKY3DS is perfect and unrecognizable.
Before spending ~$90, it is indeed worth noting the fact that SKY3DS does behave differently from legitimate cartridges, meaning it has the potential to be blocked. You should not buy a SKY3DS unless you are willing to accept the risk that you will eventually a.) have to stay on a particular software version and be unable to play newer games, online, etc., or b.) have to update your N3DSXL and lose SKY3DS functionality.
With regard to presently quantifiable stability and issues, my anecdotal evidence suggests that SKY3DS is as good as any other flashcard.
He probably don't care about that and he's right because he'll probably sell it before it get blocked (if it's blocked one day), as he said he have a Gateway so it's seem logical to think he'll use his Gateway on his N3DS when the support will be released (if it's released one day).
And as I said, he doesn't need to update his sysnand, and he'll probably won't update it anyway because I suppose he'll want to keep his N3DS on 9.0/9.2