I bought a used BD road Bike. A bandmate of mine toured on one. And I know a few folks who have owned MtBs from them over the years.

They have all been perfectly fine, I don’t think anyone regretted the purchase. All but my friend who owns the touring bike eventually moved on to something nicer, but they were solid bikes to get one into the sport, and not hold you back. Definitely not BSOs.

The mtb frame designs (geo and suspension) are almost always a bit out of date. But heck, so are a lot of older high end bikes out there being ridden at very high levels.

You do need to go over the assembly, in particular and bearings that are preloaded (headset, cup and cone hubs) as they often over tighten them.

I think the issue with bikes direct is that if you’re going to buy one you need to know something about bikes to be able to make sure it’s assembled right, and also need to be comfortable doing your own wrenching . But many people who know enough about bikes to do those things have been riding long enough that they may be more particular about what they want.

All that said.... I am bot crazy about that particular bike the OP is looking at. The frame design is fine, albeit a little dated. But the specs on it - especially the fork - are lacking. For that price point, I would be looking at one of their hardtails.

I would not bother dumping money into converting the drivetrain on this bike. Just ride the bike as is. Eventually you’re probably going to realize that you want to upgrade the fork, the tires, the rear shock, the drivetrain, and put a dropper post on It. And by the time you do all that, you could’ve just bought a nicer bike to start with The most expensive way to get a nice bike is to buy a cheap one and upgrade everything.