funny you mention haunted mansion. I had 4 doombuggies fail badly in one of my last days.
they failed, and failed, and failed.. they had to put a mark with fluorecent marker so they knew which ones to skip in the queue.
There were some lengthy waits until one was "broken" to let the kids and parent free from the protection mechanism that locked them inside before leaving the loading area.
the first day was perfect.. nothing failed.
as for CMS, I have a friend who works at Epcot, and he says he absolutely HATES MM+. Fastpass is a nightmare to manage and reservations too.
if a single system goes down, it takes the entire resort system as well.
I seen no less than 4 crashes on the 15th of feb 2015.
one of them wiped my FP+ clean.