That Libre Mercado story is from 2015, when the line was still incomplete. It says "the final costs may reach $16.6B", but doesn't mention how much has already been spent or how realistic that upper-bound estimate was.

The OP article is from 2017, and likely has more up-to-date numbers (they claim a final construction cost of $6.9B). It is true that there were major cost overruns with Barcelona's Line 9, but not to the degree the Libre Mercado article implies.

More generally, readers using the Libre Mercado article as a source should be aware that Spanish media is extremely polarized on the question of Catalan independence, far more so even than current US media polarization. The Libre Mercado article is a product of that environment and reads as a hit piece on the pro-independence Catalan leadership.

On a personal note, I'm back in the US now but I used to live on a Barcelona L9 stop and the line is really nice. Fast, quiet, frequent service, ergonomic station design—OP is right, more cities should be copying it.