Currently Yousician Ukulele has eight complete levels in Lead and Rhythm paths. More levels are to be added in the future.
If you're after harder songs there are lots in the Songs section that should give you a serious workout. Please check here how to search for songs. There's usually a level 8 weekly challenge song so that should be a good challenge as well.
Remember that if you have finished a path, you can still go back and work on another one!