Haven't done the newer ships, so can't comment on them. The Magic however? Didn't run into any big things with my powerchair. It is important to know there is no pool lift, as not knowing can be very disappointing for some.
There is one door out to deck that doesn't have an auto push button, but you'll very soon learn not to use that backdoor but use the other door instead that DOES have the auto button. Some lips, but nothing that is of "interest" with a proper powerchair. Easy to navigate without any problems.
If booked a verandah, be aware that the door is VERY heavy to open, odds are you will not be able to do that yourself. By the way, you do have an accessible stateroom booked, right? If not you are in a serious problem

. If indeed booked; you'll be fine. I've done a cat 6 acc. stateroom with my friend. I use my powerchair to get around, had my manual chair with me in case of troubles and friend uses a manual chair with powerassisted wheels. We had PLENTY of space to move around between all of those chairs. Only thinking point would be moving passed the footend of the bed, since it is only as wide to fit 1 chair and no passing traffic.
Restaurants, theater, you name it, no problem where-ever. Including public bathrooms. I've been in one that I still call "the ballroom" so much room it had. One could've done a 360 with an
ECV in there. With seating arrangements; by all means communicate what fits YOU. They're more then willing to work with you. For instance, we had a 8 person table for the 5 of us (me, friend and a DIS-family). This left enough space for not only the chairs, but also gave me the option to best suit my needs. My chair is much better suited for me seating, so I would stay in there as much as possible. It is too high for the table (caused by some extra adjustments I have on the chair) and would make eating uncomfy. So for eating I'ld transfer to a regular chair, eat and transfer back.
First night they looked at our turning radius, quickly made space where we needed it and since then the space would already be there before we'ld get into the restaurant. Not sure about all of the restaraunts but I do know for sure that at Lumiere they had to move some things around (first night). This was already altered on our 2nd and 3rd night there to accomodate properly.
At
castaway cay; be warned that there is no access. entry onto the adult beach. So you'll need someone to help you out with one of the beach wheelchairs if you want to actually get onto the beach beyond the entry.
Hallways; most stateroom hosts were very aware of not blocking it with their carts. Within no time not only our stateroom host would know we were using chairs, but all the other stateroom hosts in our hallway knew and all ver soon learned our patterns to be prepared for us to move around freely. Worst moments are when baggage is brought to staterooms after embarkation and the night before disembarkation when baggage is out in the hallways. Guests were a bigger problem in this than CM's. Have to say though, there would always be someone around already or within no time and get right onto moving stuff to make space for us.