Can't help with the Watson batteries personally but I saw a Fuji battery review that included Watson and they came out well. They are not available over here in the UK but Patona Platinum and EX Pro White were and I've got some Patona Platinum fot my Fuji XT-2 and found them very good. IIRC they charge to 95% of the Fuji batteries and last about the same time. I think that the Watson gave the same figures.
Just found the review!
The original Fujifilm's batteries for X system camera are expensive. That's the reason why there are so many alternatives available, but it also makes the choice of the right one tough. They not only vary in
ultralightphotography.net
Regarding battery grips, I could not face the eye watering cost of the Nikon ones as I don't use grips very often. Confession time. I'm not a wildlife shooter. Correction, I'm not a serious wildlife shooter.
So when I got my D810 I looked around for a third party one. I had a Chinese one years ago for my D300/D700 and it only cost 24GBP! True, it broke after just over a year, but at that price I could buy several replacements and still not spend as much as the Nikon cost. Back to the D810. I settled on a Pixel/vertex grip at a fraction of the Nikon price and it fitted like a glove and worked just fine. Some time later I was given a Nikon grip and I can honestly say that in looks, feel and operation I could not detect any difference in the two grips. I guess that the Nikon one was made from better materials but had I'd not been given the Nikon grip I would have been very happy to continue with the Vertex/Pixel one.
Fast forward to 2018 and my D850. Where do you think I'm looking for a grip? Yep - the Pixel/Vertex one was 106GBP v 400GBP + back then for the Nikon one. so I had no hesitation in buying one. On the D850 it is not quite as snug a fit as the D810 one. There is about a 0.5mm step where the grip meets the body. Does that bother me? Not in the slightest. The box is now buried in my loft so not accessible, but I thought it came with the fitting that allows the use of the higher power battery, but I might be wrong. It is not mentioned in the Amazon ad:
https://www.amazon.com/dp/B078HY919X/?tag=backcogaller-20
Ignore the high shipping rates. amazon knows I'm in the UK!
So after more that 2 years how is it doing? Just fine. I'd recommend one without hesitation. the only thing to check out is does it ship with the EL-18 (is that the right one) adaptor? If not, can you get an adaptor that fits. The AA battery trays have been an interrchangeable fit in every grip I've had, although I've never used an AA tray in any of the grips.