On the face of it, a linear genset really does seem to be the best of all worlds. My impression is that it's a matter of just implementing it, and the time to develop it. It hasn't been all that long since batteries were commonly big enough to provide all the power needs of a vehicle, and this would best work as a way to recharge those batteries, while the batteries remain responsible for motive power.
That said, I have been reading about Mazda's newest genset, and it gives me pause as to whether automakers will even choose (for mainstream) to pursue high efficiency gensets. Mazda's pitch appears to be that, most driving is local and short-haul, and eventually all vehicles will be hybrids of some sort, and have a battery of some size. Batteries are rapidly falling in price, size and weight. For a passenger vehicle, this essentially means driving on the battery most of the time, with access to a charger every night. The point of a genset (according to Mazda) is not really to provide the energy that is used to move the vehicle on a regular basis, but to extend range in those less frequent cases, such as a road trip, and as such their efficiency is relatively unimportant.
Mazda has chosen to develop a rotary for this. My first impression was "are they insane?", given how inefficient and polluting rotaries are. Their reasons, however: rotaries are compact, lightweight, power dense, and smooth. They take up very little room in the vehicle, and their emissions and efficiency are near irrelevant if they're only being used occasionally. So, rather than a vehicle having a 500 mile battery, give it a 100 mile (or whatever is deemed appropriate) battery and a small rotary genset. The vehicle will do 98% of its driving on the battery, the genset will not cause significant compromises to vehicle packaging, and the driver will not feel any range anxiety. Similar in concept to how GM advertised the Volt, and to how BMW designed the i3 REX.
I have personal interest in efficiency, but Mazda seems to be posing the question, "To what end, and at what cost?". Possibly a linear genset could be more compact, cheaper, and more balanced than even a rotary. But, it does need to be developed.
EDIT: A picture of the size of the MX-30 drivetrain. It is 168hp, and weighs less than a typical gasoline engine alone. Mazda choose to attach a rotary, and to equip it with a 130 mile (209km) battery, which weighs about the same as two full tanks of gasoline. The total drivetrain including the EV battery weighs no more than a conventional engine, transmission and full gas tank.
