Combat XP is split between heroes but not other units. Quest XP is not split.
XP per battle depends on the level & strength of the units involved. Stronger untis attacking weaker ones don't get as much XP.
Even without the split LH gives roughly 20% less XP per battle than FE did. However the last tweak of the beta was to increase the number of "end game" monsters and quests so that you could still level up even with the reduction.