> However, if you have a very cheap mobile data unit price (less than €3.85 / GB in 2017), your operator may apply a "fair use" limit for data that is lower than your domestic allowance when you are roaming.
Fair use still means more than nil. AFAIK, this was included to avoid that people buy cheap plans in countries they don't live. For all plans I've seen so far, the allowance was sufficient to cover frequent travel, just not enough if you're only abroad.
In France they didn't, what do these "European" plans do? Since you are legally allowed to use any local plan when you are abroad (except if your plan is less than 3,85€/GB indeed, then your operator can decide to give you less data or charge you more within this limit).
I'm just using my regular 10€/40GB plan whenever I am abroad and it works fine.