I visited a different sort of "Cathedral that failed". In Manresa, Catalonia there is a basilica named Santa Maria de la Seu. https://en.wikipedia.org/wiki/Santa_Maria,_Manresa

My fiancée, acting as my native guide, informed me that Manresa was once in competition to be the seat of a bishopric, and this grand church had been constructed as part of that bid, perhaps in the 9th century or so. It is sort of like how nations build Olympic stadiums in hopes of attracting that sort of honor.

Anyway, the bid did not succeed and the city remained within the jurisdiction of a more ancient see at Vic. https://ca.wikipedia.org/wiki/Bisbat_de_Vic

Nevertheless, Santa Maria is a gorgeous sight, with flying buttresses and marvelous stonework. My impression from the visit is more of a tourist attraction and museum. We did not attend Mass at this location, if indeed it is held there.

We did, however, attend Mass at the Sagrada Família. In fact, I attended Mass at two distinct Sagrada Família churches: there is one (slightly less grand scale) in Manresa too. Unfinished and unconsecrated as it had been in 2008, Mass was not yet celebrated at the main altar, so our liturgy was a rather intimate experience in the Catalan vernacular in a side-chapel. (Unfortunately, we did not have time to tour the whole thing. I should've liked to see the crypt, the breathtaking main interior, and ascend one of the towers.)

We were also on a budget. When we arrived at the Cathedral of Santa Eulalia (there are a lot of cathedrals, basilicas and churches in my stories of Catalonia) we were confronted with an admission charge. The Catholic faithful and guests are not charged to attend the sacred liturgy, but tourists are charged to be lookie-loos in the off-hours, and since we ignored the schedule of Masses, we were lookie-loo tourists, and the very thought of needing to spend money offended my fiancée, who considers herself a faithful Catholic woman. I consoled her with a gift shop visit.

While the Cathedral of Santa Eulalia is definitely a "finished work", you can see nearby an ancient Roman wall, which is of the type that were probably always under construction or subject to improvement by the occupying forces in Hispania.