just some provisional thoughts about that:
It is a very interesting question whether Sonic Pi is suitable for which music and what specific workflows and learning objectives it encourages. Having said that, I do believe that this question has yet to be answered, which will involve time and people to find out (the forte piano has had more than 250 years so far). IMHO these questions should rather focus on the potential than the limitations or - to put it differently - limitations usually trigger creativity. Every instrument has limitations (such as that you can’t play a glissando with a piano) or rather ‘character traits’ which shaped the culture and creation of music made with it. To be more concrete:
Playing by ear is a complex skill which can be developed via different paths. I e. g. made the experience that working with Sonic Pi while trying to recreate sounds and arrangements is a very interesting and fruitful way to develop one of the most important aspects as a musician, which is to listen very closely.
I also think that Sonic Pi and its ‘code approach’ can be very helpful in analysing music on a somewhat deeper level, because it forces you to find abstractions (in a way ‘metaphors’) which deviate from the construction rules of what you have been analysing (or recreating) in the first place. It is a bit like translating from one language into the other.
You are right: It is not easy to create melodies with Sonic Pi but I am not sure that this is only a limitation. Meanwhile there are some libraries available which try to overcome this ‘shortcoming’. I myself have been experimenting with my livelooper to be able to quickly create melodies (e. g. a bass line) on the fly and furthermore: to start manipulating and extending these adhoc creations as samples.
By the way: Messing around with samples and effects is a lot of fun and will eventually always generate interesting results. For me it has a lot to do with the appropriation of sounds which you transform into something which starts belonging to you. Even better: Record you own sounds and work with it (this does not have to be music!).
‘Repetitive’ music (it has still to be defined what exactly that is and where the variations ends and the repetition starts) might not be everybodies cup of tea but it largely dominates the musical world we live in. I definitely would encourage (and I do so in my courses) to have a closer look to the highly designed musical products our world is abundantly filled with and find out what you like, why you like it, whether and how the concept of quality applies to these products and what their aesthetics is about. This is - again only in my personal opinion - best done if you leave the position of the consumer and start producing yourself. Again this is about appropriation in the best sense of the word.
I do also think that Sonic Pi deserves a prominent place in the field of musical education precisely because it is somehow different but nevertheless accessible. I can not validate this empirically (not yet) but I am sure it brings people into creating music which otherwise would stay on the mere consumption side.
So far some thoughts about the intesting subject you broad up.