What an odd question!
I've sat here for 5 mins thinking, then couldn't come up with anything so had a look in my oldtextbook...I really am stumped. Part of me wonders whether the question is worded correctly, and whether they are not really asking what they mean to ask. The only disadvantage I could think of was that it's not easy for clients to do it themselves, but it's a silly one really. I did have a laugh though, I searched on google for 'disadvantages of IHM' and up came a review site...someone put for their disadvantage: you smell like vinegar after!!!!!
Oh dear, not getting very far here are we? Lets hope Gussie can come up with something.