I thought I would mention the result of a test I did, since I found the description unclear. You CAN in fact, use Rapid Shot with spikes in either humanoid or manticore form.

This archetype is great, but very buggy. The spikes you throw have such a fast animation speed, that unless you're aiming at someone fairly close, you will waste attacks because the second attack will be launched before the first attack lands, even if the first attack is going to kill the target. So you'll waste attacks on targets that are already dead. Grow Spikes also doesn't work properly while in Manticore Shape (or at least what is displayed on your character/inventory screen doesn't take into account the increased damage die or crit multiplier from Grow Spikes, unclear on whether or not it's actually working in practice).

Despite all these bugs, still one of the best Shifter archetypes, I highly recommend multiclassing with a class that can get Point-Blank Master like fighter, slayer, ranger, etc. and the going Legend.