To resume :
- is a heavy javelin : throw (twice), pick up later if able, high(ish?) chance to get damaged on impact
- great at piercing & disabling shields
- likely useless at melee, sword & shield would have been used for that after having thrown both of them
- uses a lot of expensive iron (steel?)
I mean, between the 2 pila used by the first two front lines of their heavy infantry compared to the 7 javelins of their light infantry, maybe ? The author does insist on the expense.
Also, this was to compare with RimWorld, where pila are made with wood, and wood only. (While IRL your typical spear would have less metal and more wood than a pilum ?)