They were initially designed/made by 2 different companies in 2 different countries ~17 years apart. In its original form, the design of the BMW reservoir is very different; only after modification does it appear more like the X1/9 reservoir. The only thing similar between the 2 reservoirs is their exterior length & width, which is the main reason why they're a good replacement (after modification) in the X1/9.Since it would appear that the BMW and FIAT ones are made by the same company based on the similarity of the design...
The X1/9 reservoir is hidden under the rear corner of the hood, away from direct sunlight (UV). It's the thin & poor quality plastic used that's the main issue with these. The BMW reservoir is definitely superior in this regard, especially considering the engine (bay) heat it's normally subjected to in its native use....it's likely the placement of the reservoir has a lot to do with the longevity. The X reservoir is subject to UV light while the BMW is under the hood and is almost never exposed to light, and as we know '70s and '80s plastics does not hold up to UV light all that well.