These guys seem to think relational databases are a valid alternative in the process automation world, and seem to have made a successful business around the idea:
SCADA systems usually handles small amount of data (because data comes from real sensors, this sensors is expensive and you simply can't have millions of them).
yeah. I actually suggested that to someone who sells an industrial historian and his response (believe it or not!) was that no, there is too much data, a relational database solution is no good (and by the way! the business model of inductive automation will never work!)
I'm sure writing would be no problem, the only thing I wonder about are complex (aggregating) queries in situations where you care about delays (interactive visualization perhaps).
https://inductiveautomation.com/news/process-historians-vs-s...
[I have no data on how well their product performs in practice]