I don't think you can say that any bb is too high or too low - it just depends on what the bike is intended for and how you have it set up. The distinction between bottom bracket "drop" - or distance below the axles of the wheels - and the finished "height" - or ground clearance - both matter equally, and it's hard to know what is making the difference for me since different tire diameters make it an apples-to-oranges comparison.
When I got my bought my Clem, I had visions of using it as a mountain bike. That's how the model was originally conceived, in fact. It really didn't work for me that way because I didn't have enough standover clearance and I would regularly high-center on obstacles, so it kind of organically evolved into more of a road-ish, town-ish, smooth trail-ish, super-fast pathway cruiser. It was fantastically comfortable - largely because of the generous, rivendell-typical bottom bracket drop - and it quickly became one of two bikes I ever wanted to ride. Just not as a mountain bike. Prior to that, I hadn't known I NEEDED a bike like that.
When I got my Susie, it was with the intention of incrementally improving on my Clem, but keeping it for the same type of riding. In other words, that same every-day bike I hadn't previously known I needed. However, this model was ALSO very much conceived as a mountain (hill) bike and, in this case, Grant and crew consciously made it more purposeful... and used a higher bottom bracket / less drop. Presumably for clearance to avoid pedal strikes.
Combined with my ginormous tires, I can't under-emphasize how much I feel the difference between the two bikes. That's my main point - not that one is bad or one is good. I feel like I'm perched on top of the Susie, while I rode inside of the Clem.
The low, stable feel of the Clem is what made it and many other Rivendell models feel so comfortable and stable. If you're coming from "normal" brands' bikes or older norba-era mountain bikes, the Susie will still feel low and great. In my case, it so happens that my 3 other regularly-ridden, true "mountain bikes" all have even lower bottom brackets than the Susie. Like the Susie, they also have much slacker seat tube angles than is vogue for most mountain bikes these days. But they're not typical. Those models and my own tastes were both influenced by appreciation/preference for Rivendell's design philosophy. So ironically, this personal experience makes my Susie feel high too.