I think this is less the case now, but a lot of the early "gravel" bikes were rebadged CX frames, which is makes for a different bike than a rebadged road frame with more tire clearance. Newer gravel bikes are also getting flared bars, dropper posts, mounts galore, slacker HTAs. Not massive differences, but often different enough.
Gravel bike vs hardtail mtb is dependent on terrain. On smoother, rolling terrain, gravel bikes are surprisingly (much) more efficient - and with some 40s, they're also way more capable than you'd think. It's hard to understand how significant the difference can be without riding a gravel bike, or riding a hardtail with a group of guys on gravel bikes. Even at very relaxed, fat mountain biker pace, you work so much harder to keep the mountain bike up to speed. That advantage starts to go away as the terrain gets rougher and more steep. Only the rider can determine what the threshold is to decide one over the other, and no amount of e-research is going to tell you what works best for you.
There are plenty of big, sketchy, packed dirt and gravel roads around here, but they're 10-15 miles away over mostly fast, rolling terrain. I'll drive long distances to ride my mountain bike, but driving to ride a gravel road seems crazy. I'm willing to suffer a little more on those steep, gnarly climbs and descents in favor of a faster, more efficient approach and return trip. Also, I'll be honest, I kind of like the change of pace and challenge. It provides a nice alternative to mountain biking during the winter months. FTR, I'm talking gravel roads here. There are some short dirt road sections that are fun in a novel, holy shit kind of way, but I'm mostly riding maintained gravel roads. They get washboarded and rutted, but then they get new gravel (sketchy AF) once or twice a year. If I was riding lots of unimproved dirt roads, forest service roads, double track, that kind of thing, I think I'd probably opt for a mountain bike.
