Some years ago I studied Uechi-ryu karate, and sanchin is the first kata. Seems very hard to me, but I only got to shodan (first black belt). Uechi-ryu was brought back from China by an Okinawan and tradition says he learned PanGaiNoon there and PanGaiNoon means half-hard-soft. I never got to the soft part, much less the internal, but you might direct your question to the PanGaiNoon site (Google it).
BTW: After I got my black belt my teacher (a yondan at the time) suggested my style was too hard and I should try tai chi, so I don't think he saw any softness down the road, or internal stuff (maybe I conflate them, am I wrong?).
Geneh