My initial thoughts and findings on Biospheres:
Very often you're leaning out over the void. Tape that shift key down. (Q: is there a run/sneak toggle mod, perhaps using Capslock?
Since you can't build down, and resources are often near the bottom of spheres, it's rather important to dig down at the center of a sphere, then extend a floor out from the lowest block you can find.
(Has anyone ever tried building downward by placing a water source pouring into space, allowing the stream to carry you down, placing blocks as you go, then swimming back up? Scary idea.)
It's crowded. Since (at a guess) 7/8 of the horizontal map squares don't have a single valid spawn point at any height at all, the spheres fill up very quickly with all types of mobs. I killed off some squid in a pool, and within 10 seconds more had popped in. It's much the same for hostiles. If you leave even a single block uncovered by torchlight, a monster will spawn there during the night.
It's probably a good idea to wall off all bridges in every sphere you visit, just to prevent movement of mobs along them. Even the peaceful ones can headbutt you off the bridges if the siderail placement is bad.
Spheres where ghasts spawn should be treated with caution. Do not let that glass break, else you'll have ghasts floating everywhere. (Hmm. climb up the outside, break holes in a 5x5 pattern, drop gravel down, preventing spawning? Probably not enough gravel in the map.)
Weather is funky. I saw that the mod has special code to deal with weather, but I haven't seen any examples yet. All I've seen is rain & snow outside, on the bridges, with no weather effects inside. I hope there is per-sphere weather.
Trees are rare. But wood is common. It's just dangerous to obtain.
It's not compatible with MoreFlowers. It doesn't crash, the new flowers just aren't placed.
It's not compatible with IndustrialCraft. Almost, though. Everything works, just the three new ores aren't placed when chunks are created.
What I did, that looks like it works, is use NBToolkit to change all iron ore to tin ore, then gen iron ore again, then change iron ore to copper ore, then gen iron ore yet again.
I didn't try to get uranium to place, and if I did I'd probably randomly change some of the diamonds to uranium instead, making it even more important to build out to the small obsidian spheres.
And if I decide to play a serious Biospheres/IndustrialCraft map, I'll start with some naked speedruns to expand the map size before I mess with ore placement. Heh, maybe I'll grab a 'naked' skin when I do my streaking.
I'll also install that torches-anywhere mod and light up the domes of my claimed spheres.