Each EMH experience can be different.
I can say, from personal experience, that they CAN be
EMPTY, or FULL or somewhere IN-BETWEEN for attendance,
often within the same week.
PM EMH's are a mixed bag. I've been when they were
PACKED... but on other nights (sometimes within the
same trip), the crowds were basically so low that it
was like "free rides" on all the open attractions.
Those who have attended PM EMH's and say that "they
are TOO CROWDED" are right... but ONLY for the nights
they may have attended.
I can say, from personal experience, that they CAN be
EMPTY, or FULL or somewhere IN-BETWEEN for attendance,
often within the same week.
------------------
We went to PM EMH at AK on a Wed night and PM EMH at MK
on a Sun night (early Spring visit.)
The EMH's were so EMPTY that we could have stayed on many attractions and ridden around and around.
This included Splash, BTM, and Pirates (and many more) at
MK, and Exp. Everest, Kali, and Dinosaur at AK. And
the crowds during the DAY at these parks were very
LARGE... within the first hour of EMH, they simply
"evaporated."
But, it can change so much (either way), that saying
EMH was "like X" on a given night, can be meaningless
on another night in the same time-period. So, you
simply don't KNOW unless you go.
The rule seems to be, it is far easier for guests to
STAY UP at the end of the day, than it is for them to
GET UP at the start of the day.