White rage, Muki tensei, Tayuya sound gen, Kidomaru's webs, Dance of the seedling fern, are all far less effective outside of a cave. Especially the first 3. The only good thing kabuto has is his near immortality(healing + oral rebirth).
Outside of a cave, most, if not all, top 15 would simply destroy him even before he enters SM. Anyone who says kabuto enters SM while moving/having a jutsu activated is a fool. Not even jugo has ever done that shit.
SIDE NOTE: Manda 2 is also overrated. People keep spouting crp about manda 2 taking down turtle island, when infact, all manda 2 did was wrap around it and dive. They mention that part as evidence for the overration, but what they forget to mention is the leverage deidara gave it with his massive exploding Squid. Manda 2 can't take down turtle island on it's own, nor is it some super power summon. It's just a little bit better than manda like the manga said...