A lot of interesting points being made recently.
Indeed, content design plays a very big role in how strong/weak a job is regarded:

(This list refers to balance at the time of release)***

- 1st Coil:
T1: SMN - Multi-doting and ability to kite slimes.
T2: Balanced - SMN had some utility in Supervirus, due to M-DMG enemy attacks.
T4: Balanced - A mix of AoE and Split-enemy placement; a very balanced fight.
T5: SMN - Pet soaking, Miasma > Lethargy, clutch battle raises were a thing.

2nd Coil:
T6: SMN - High-movement and RNG mechanic choices hinder BLM.
T7: SMN - Kiting & multi-doting.
T8: BLM - Very high uptime, no mp issues.
T9: Balanced

Final Coil:
T10: BLM - High uptime, no mp issues.
T11: BLM - Same as above + resistances hinder SMN multi-doting.
T12: BLM - This is the closest turn to being balanced; SMN has utility in Bluefire, but BLM raw damage and on-demand burst wins.
T13: BLM - High uptime, no mp issues. Also, Apocatastasis is huge here.