I have dual-sided Vector 3s purchased in January 2018 from Clever Training. I started out with issues, but did the homework, requested the new battery covers, and put a drop of baby oil on the batteries when I replace them. I’ve had no issues since. Very few dropouts, maybe once a month for a few minutes out of a workout, connecting via Bluetooth to either TrainerRoad (iOS) or my Wahoo Elemnt Bolt. Overall, they’re not as reliable as my old PowerTap hub was, but I like the ability to easily change them from my road bike to my TT bike, and I like the ability to race with power with my race wheels.
I think whenever I buy a new bike, I will go with a crank-based power meter on that bike, but for now the V3s suit me. I opted for them over the Assiomas because the Garmins are made in the US, so I figured support would be easier, and I didn’t want to have to mess with recharging the pedals, opting instead to replace batteries every few months. But everything I’ve read about the Assiomas is that they’re just as good, if not better, than the V3s.
I’ve also thought the dual-sided was worth it, but depending on your budget, the single sided is probably just as good 90% of the time. I wouldn’t get wrapped up in all the cycling metrics that Garmin markets. For the most part, they’re pretty worthless at this point, and any power meter will be able to give you the metrics that matter. I like the R/L balance metric, but that’s not unique to Garmin, only to dual-sided meters.
For battery life: I replaced the batteries before I moved in to my new house, so in October of 2018. Since, I’ve completed SSB1 MV, and about a half-dozen other TR rides. I’m well over 40 hours of use on this set of batteries, and according to TR, they’re still “good”.
Also, no issues on my limited wet rides.