IDK if anyone really answered -this- question properly, so I will.

> Isn't that exactly what hybrid cars (e.g. Prius) are? Extremely efficient gas engine for highway cruising, but insufficient for acceleration, which is aided by electric motors?

Well, it kinda depends. Where Hybrids get the biggest MPG boost is in city driving. The stop and go traffic lets you use Regen braking and go quite some time without the engine kicking back on while still moving forward.

I still don't understand Honda's system enough to speak well on it, but I can speak to THS because it seems to be the cheapest to do and is most proven on the road.

The general parameters for a THS type system on a Toyota Prius or Rav4, or a Ford Maverick/Fusion/Escape is a 2.0L or 2.5L (at least in modern US examples) engine paired to a simple planetary gearset containing a power split device. It is a single speed (At least in the cheap configurations, however that simplicity is possibly close enough to be viable for air usage vs a reduction gear.) That's part of why they tend to have fairly large engines, the valve timing magic gives them at least a bit more HP to not be too bad on the highway.

> but insufficient for acceleration

Going back to the modern cases, the engine is typically sized large enough to give some acceleration even on the highway. Not always great but usually enough.

At least as far as the non-plug-in hybrids, the 0-35MPH can be surprisingly peppy.

The bigger magic (again, at least as far as THS) is it makes it easy to just run the engine at the 'most optimal RPM' for certain tasks, excess energy gets piped to the battery or back out through the system, this does also help reliability tho, because you can then design the reliability of the engine around certain RPM ranges...