Post 9 of the disABILITIES FAQs thread is about Universal. It includes direct links to the Universal Orlando website, ADA information and Rider's Guide that was mentioned by a poster above.
You can find that thread near the top of this board or follow the link in my signature.
Two things to be aware of about Universal:
NONE of their lines are
ECV or power wheelchair accessible. Anyone using powered mobility devices will need to transfer to a manual wheelchair to be in line for any attraction.
This apparently includes things like Olivanders to some of the staff there because they consider it an attraction, rather than a shop. It is not listed as an attraction in the Rider's Guide.
I noticed on a recent trip to IOA that many of the shops were quite tight and would be very challenging to get around in with a wheelchair -especially those in the Harry Potter area.
MOST of the attractions have at least some warnings. Some seem silly - there is a whole list of warnings for theHigh in the Sky Seuss Trolley, which is basically a very slow elevated train ride.
But The Cat in the Hat, which has similar warnings is VERY rough and whips you around quite quickly and unexpectedly.
Neither Universal park (IMHO) is particularly accessible for people using wheelchairs who need assistance from their party with transfers. The Universal parks, especially IOA, are more thrill oriented than the WDW parks, so anyone with limitations on how rough of rides they can do will find less to do at Universal.