In my opinion Nekima is worth the SS, but only in a grow list.
Last night I decided to make a grow list for nostalgias sake (haven't used one in aaages). Before the end of the first round I had 5 Young Nephelim flying around. Sure it means I spent the first round growing instead of moving across the board, but it also meant that after that I had 5 quite mobile creatures that can charge over intervening terrain and models, have melee expert and on top of that auto-trigger Flay, making their damage output quite insane.
Nekima herself didn't do a lot that game, she might hit hard but I found her far too vulnerable. Still worth the 13 points though.
Don't get me wrong, I'd love to see a more versatile version of her, or on the flipside a henchman version of her (they tend to be a lot less versatile but stronger for it).