Well, you're begging the question in favour of some interpretations of that mystical experience of unity. Granted, Buddhists wouldn't call nature "monstrous." That's my interpretation which derives from other philosophies such as existentialism and from modern science. So I'd be saying only that in effect, Buddhists are neutral towards a monstrosity. Whether Buddhists agree that nature is monstrous isn't the point. On the contrary, that's a point at issue here.

The early Buddhists were naturalists, so what should we make of nature as we presently understand it? Has modern science added to our knowledge in that respect?

I've had some peak states of consciousness while high on cannabis. I wrote about them, and they were powerful at the time. I felt like I was speaking to God. But the best way to understand that kind of experience is to naturalize it, which deflates it. It was just one part of my mind talking to another part while my defenses and inhibitions were down, so it felt like impossible-to-resist revelation.

I suspect something similar happens with respect to mystical experience. I have no doubt the experience is subjectively real. But what's its philosophical meaning? What are its metaphysical implications? I don't think early Buddhists would care much about metaphysics, but in that case they have no grounds for denying the attribution of monstrousness to the whole of nature. The Buddhist would say only that the mystical experience causes him or her to be at peace, given how human psychology is supposed to work.

Indeed, I'm writing something next on ataraxia (mystical or enlightened imperturbability) vs existential horror, so I'll have more to say on this soon.