I did some testing with 2 oUFs running. oUF 1 uses OnUpdate polling and oUF 2 uses the new frequent event(API-6 branch of oUF).
First thing I noticed is that both UNIT_HEALTH and UNIT_HEALTH_FREQUENT shouldn't be registered to the same frame.
(line breaks every sec)
oUF #, Gametime, eventname, # of updates
with just UNIT_HEALTH_FREQUENT in oUF 2
OnUpdate gets called once after entering world, then identical after that.
Comparing UNIT_HEALTH to UNIT_HEALTH_FREQUENT
I see an about an avg of 250 ms delay on UNIT_HEALTH when damage is taken. FREQUENT gets called a few more times and when idle (just health ticks) their identical.
This is just testing one unit is each oUF. Haven't done anything with multiple units.