Haven't rotary (Wankel?) engines proven to be unreliable? I love the RX-7 (my dream is to own a FD one day), but IIRC even the more refined Renesis engine on the RX-8 seems to have been proven to be ornery. I think Mazda has a rotary acting as a range extender in their MX-30 EV. I wonder how that will fare.
Ok, hyperbole, a bit. But for reals, check out the man himself. It's pretty wild.
From an engineering perspective, there's some hard limits. Heat is going to always be a problem for Wankels due to the low thermal mass of the thing, and, of course, the fact that there's always a power cycle. The actual rotational efficiency is pretty bad, combustion impulse goes towards the center of rotation, some of it goes backwards. Compression is inherently limited from the necessities of spinning a triangle in an circle. It's nearly impossible to lube the thing effectively; early on they tried to intermix oil and fuel only to create a high-revving Soviet-era ecological disaster because Wankels tend to spit fuel before it's done combusting.
I love, love, love cutting Gordian Knots like this, which makes me want to love the Wankel, but at the same time, sometimes Gordian Knots are tied for a reason. High flow, high RPM, low weight, yes all super awesome, but . . honestly, that's called a turboshaft.
I dig this Wankel Rebooty from the Liquid guys, but they also seem to have killed the one solid advantage of the rotary design: its low weight. The thing's a brick. Also, that wobble it has when the rotor engages, do you see the back and forth? That's some weird oscillating vibration there, and I couldn't help noticing what looks like some wicked counterweights. Of course that could be some other doodad I'm ignorant of.