User blog comment:Hayashi H/Max-efficiency Map-clear Formations/@comment-33019381-20160325010424/@comment-26486243-20160405092755

The Ro.43's bonus is firepower and evasion, which makes it important in builds that need the firepower to do well and evasion to survive, which tend to be CLs. Most CAs are already over the needed firepower to sink their targets and all BBs are at firepower cap, so those would benefit more from the higher artillery spotting rate afforded by the Type 0 Obs seaplane than firepower, while they tend to resist damage more from armor than from evasion.

They're almost identical tier though, so this is really being very picky. Aside from BBs that still should not use Ro.43, both CAs and CLs can be considered optimal with Ro.43, Ar196 Kai or Type 0 for the most part.