feats are crap measures of relative strength. We have few feats, in part because of budgetary constraints on a show.
Also, not having feats because feats aren't attempted is very different from having a lot of 'failed' feats. There are a few 'failed' feats, like struggling with chains, but a lot of other very good feats, like punching through concrete or brick.
The beatings Buffy stood up to from Glory, for instance, judging by the foot stomp feat, could well put Buffy's durability in the ballpark of the wrecking crew...now, I don't buy that, but that is in part why its so difficult.
The only 'concrete' information we have is that vampires are stronger than people in general, and that Buffy is significantly stronger than Spike and stronger than Angel. Other types of vampires are stronger than 'normal' vampires too.
We know that 'enhanced, super-soldiers' aren't on Buffy's level too, but how do they stack up.
By her highest end feats, she's probably above class 5. By her failures, maybe class 1. By lack of attempts, we gain no information. Her feats that put her at 1 ton or more are at least as numerous as those that put Cap in the 1 ton ballpark, with FAR fewer showings.
So, in terms of probability, she's almost certainly above peak human. Likewise, she's pretty much statistically certain to be less than class 10, and pretty unlikely to be at or above class 5. Most likely somewhere in the 1-4 ton range, and good arguments can be made at least up to 3 tons.
If JW wrote a crossover, I'd be surprised if Cap were stronger, for instance, but I'd be VERY surprised if she were stronger than Spider Man. I wouldn't be surprised if she overpowered Vermin (at 2 tons) however. I'd be a little surprised if she overpowered Beast, who has ranged from 1 ton to a 10 ton listing in at least one handbook, and often considered about a 5 tonner. That said, the version listed at 10 tons has similar leaping capacity to what she has shown, and his running speed is explicitly lower (40 mph for Beast, while Buffy was able to keep up with a bus).