I've heard that they got banned because the string would "merge" with the tissue or grow into the tissue
which made it impossible to remove all of it if the patient wanted to explant/downsize/switch.
Yup, this is likely one of the key issues.
I spoke to the wife of Dr Baeke (you might recall he did Keisha Evans' huge implants) a while back, and she said she saw a video where the surgeon was trying to remove the string implants. It was apparently pretty unpleasant. The string can merge with some of the surrounding tissues, so either you cut the strings short and leave the rest in the tissues, or you try to cut open the tissues to try and release the string. Either way, it's a lot of work, and quite possibly never guaranteed to get all of it out.
Another reason for the possible restriction might be due to the unpredictable growth. Imagine if you got a new dental filler, and it was discovered that it expanded or 'grew' slightly in some people, but for a small percentage of people it grew quite radically. Oh sure, you could 'fix' it by going back to the dentist once in a while to have the excess filed back, but that seems to be a bit of an inconvenience, if you ask me.
The medical community really really like predictable outcomes. Random growth is not what would be considered predictable.