I've had numerous Gassers, Sherco', Beta' over the years (Rev 3 and Evo's) and a Vertigo. Currently on a 2021 Factory Sherco and it is lovely, incredibly light and responsive with fantastic suspension. I found all bikes to be the same as I upgraded over the years to be fair. In my opinion though and from my own experience I would say that the Beta's are the most reliable in terms of engine and gearbox.
I've had electrical failures on all brands of bikes, but the Sherco' in particular seem to begin to rattle a little sooner in comparison to others, I've noticed this from seeing and hearing the second hand bikes that mates have owned and at events. But this comes down to who has owned it and if the bike has been looked after, although I have heard of the barrels going oval leading to piston slap on the Sherco bikes. The older style Sherco with the tank and airbox switched, I had one and this was a fantastic bike (2014 factory 300), the biggest issue I had was in relation to dirt getting into the fuel pump which was easily managed by splitting and cleaning routinely and by trying to avoid getting crap in the fuel tank, fuel cap covers are available to help with this. That said, I've heard older gassers and beta' rattling away too, so these things are common to all brands over time, it comes down to maintenance and how the bikes have been ridden at the end of the day.
If the bikes ride well and sound good when viewing I'd say any of them are a safe bet if they have been looked after. You will have equally as much fun on whichever bike you choose, once setup to suit you then you'll be onto a winner regardless of brand as I believe they are all good.