Magnus is going to lose a few things: sense of touch, taste, and smell! Magnus in canon only recently got better from his soul being stuck in a mannequin body, so he's going to have all the sensations of not being able to feel anything again but this time in his actual body. Which will be super awesome and not jarring at all for him!

Also he's probably gonna drop or break stuff constantly because he wont be aware of how hard he's holding things. And his instinct to rush into danger will probably get him in a lot of trouble since he can't feel pain and won't know how injured he is still after it's all done!