I think that the concept behind the eBikemotion hub motor system was (and is) pretty straightforward: they wanted to make a system that bike manufacturers could integrate into their existing bike platforms (frames and components) fairly easily, with minimal engineering and manufacturing changes. From the bike manufacturer's perspective, this saved them time and money. This is especially important for smaller manufacturers, like Orbea, when trying to compete with the Treks and Specializeds of the world. They were also targeting higher end road bikes vs. commuter or utility bikes, so light weight was a goal.
Orbea clearly had a relationship with eBikemotion, as Orbea had an exclusive to use the X35 system for one year before it could be sold on any non-Orbea bike. Both are Spanish - eBikemotion was bought by Mahle in the Fall of 2018. My guess would be they did some joint engineering and development work, so it was a win-win for both companies. One of the reasons I purchased a Gain (in early 2019, here in the US) was because I felt that Orbea had the most experience with the X35 platform, and had probably worked out the kinks the best (
and 5000 miles later, I'm very happy with my purchase).
Also, when Orbea came out with the Gain, and Bianchi at around the same time with the X35 Aria, it was a revelation. A true road bike that looked and performed like a road bike, weighed 26 pounds or so, and provided "Enough Power" to assist a rider. The photos of the Gain sold me - it was lovely, and other than the rear hub, looked like a regular carbon road bike. No swollen down tube, no bulky bottom bracket, no unusual frame designs.
I realize that's not what everyone wants - it's not a commuter bike or a mountain bike. And there are certainly some advantages to BB-mounted motors. But I've felt every day that I've owned my Gain that eBikemotion did a great job of engineering. It is quiet, reliable, and overall, it just works.