I've ridden my KS18XL for about 600km. Haven't had any problems with it yet. The lift sensor has failed a few times. But I lift it pretty often and I'd guess it's been working 95-98% of the times. But riding it I haven't felt anything that would make feel unsure so far. But most owners, including me has only had their wheels for a month or two. And that's probably too little time to evaluate. The only problem I've heard people have is with new app and firmware. Which I haven't installed yet. When people say it works fine, maybe I will. But if it ain't broken, don't fix it. =)
I also have a V8 and I've put more than 2000km on it. But I realized after a few weeks that I would grow out of it pretty fast. It's great for short commutes since it's so light. But on longer rides you're always aware of the little battery that is gonna stop you from feeling the freedom you get from a ~1600WH battery. My choice for my first wheel was between the V8 and the KS16S. If I got to go back and change my decision I would have gone with the 16S. Those extra 5 km/h and almost twice the size battery would probably not have made me grow out of it just as fast.
I can add that I'm not at all an extreme rider. I haven't yet gotten the XL up to max speed and I feel no need to do it. But it feels great to ride a wheel that has more to give than I need to use. On the V8 I was riding the limit pretty much all the time, and that just takes the fun out of it. Don't get me wrong, I've had lots and lots of fun riding my V8. But after riding the XL, the V8 just feels like a kids toy.
When it comes to reliability I think you're pretty safe as long as you stick to the biggest manufacturers (Kingsong, Inmotion, Gotway and Ninebot). Since your first choice also was an Inmotion wheel you probably chose quality before performance, so you would clearly not chose a Gotway wheel. Only having the experience of Kingsong and Inmotion I would say that Inmotion probably is the better manufacturer, but Kingsong is not far behind. And the Kingsong wheels has way better stats than the Inmotion.
Like I said, I had regrets from buying the V8, but I'm pretty sure I will never regret my XL. It was a big change with 10kg more weight. But being a man of 195cm/100kg it feels like the perfect size for me. You just need to be a little bit more confident while making turns and the wheel will follow. I'm sure you wouldn't regret buying a XL either. I bought mine from Speedy Feet, they have the XL-footplates in store. Don't buy a XL without the larger footplates.
Best of luck in buying your next wheel!