His involvement in the plot is what would making him a companion complicated.
All scenes and plot points he appears in had to be adapted which means redoing a lot of perfectly working things.
it's not like you didn't have the exact same problem with Halsin and his transition to companion which forced a bunch of re-writes and changes to plot and character out of nowhere. But in his case people already had several elf or half-elf companions and romance options to choose from, so he doesn't even bring anything to the table that we don't already have. Barcus or anyone else could have been given the same attention and been re-written asa companion and actually added something to the party and story.
- yep, very much a waste of a companion slot. I would say though, that Barcus at least woudl have the advantage of having actually stuff to do in act 3.