Is the 68A MOS AIT very difficult?

Short answer: yes

The program is split into 12 courses encompassing a variety of equipment to help familiarize you with basic medical device concept and electronic theory.

The first two courses cover electronic theory, have the highest failure rate, are math intensive and are designed to weed out weak students.
The next ten courses are split up between classroom and lab. In the classroom you will move and a run learning as much as you can about clinical application, test out, and then quickly go over your equipment schematics in preparation for lab. You will then be tested to find bugs to the component level as well as familiarize yourself with preventative maintenance procedures.
A lot will be demanded of you and understand that studying will take up all free time you have, at least at first. The school house is great and if you show willingness to learn they will help you when you struggle as long as they don't have to spoon feed you.
Army side of things is pretty rough in that in Tradoc you are never seen as an adult and you will find it makes a difficult mos even harder. You will not have enough time in your day to study so you will have to sacrifice sleep, weekends, and meal time to survive academically. Your mos is secondary to being a soldier.
But do not let that turn you away, the DoD BMET program is one of a kind and produces the best bmets, and Army bmets develope an iron-strong mental fortitude simply to survive.
Form a good study group, make good life choices, and always have your canteen.

I was senior instructor at that school 2 and half years and yes it is difficult if you don't come with the right mind set... its 10 and half months long... and very intense ... it equates to 2 years of study on the outside of study... plus you most maintain your physical fitness throughout the training...