I have a couple of 8 string ukuleles and they do sound great, yet as someone has already said, twice the strings is also twice the fretting pressure so they can be hard to play.
A really well built/accurately built instrument can sound twice as good yet a poorly built instrument can sound twice as bad and they are more difficult to keep in tune.
A good one sounds glorious when strummed yet I also finger pick on mine but it is a fraction more difficult to pick on compared to a 4 string instrument.
As a professional instrument they do provide a certain sound if your willing to work at it yet as an amateur instrument, are you willing to put in the extra work to make them sound fantastic or would it just end up being a burden to you?