I've been waiting for this for a long time. It doesn't "look" like an engine, but it is an engine! A stirling heat engine. And the price, although high, is way more reasonable than anything else I have ever seen!!
They look a little primitive. Not always a bad thing.
They use PTFE rings, which will require a very smooth bore in the cylinder to have any decent lifespan. Without running at elevated internal pressures, the output will be more like 100 watts rather than 1,000 watts.
Running pressurized is tricky to get done in real life.
There could be lifespan issues depending on exactly how hot the hot end gets.
The cold end looks air cooled, which has pros and cons. Water cooled will give a bigger temperature delta, and could be passive. Active cooling of either kind sucks up power if not done super efficiently.
Hey, they have actual stuff for sale, with actual running engines to look at and bigger than a toy. This already puts them head and shoulders above every other Sterling engine out there.
Baxi have for some time been offering a sterling engine built into their central heating/hotwater boilers, to generate electricity from the heat your boiler is producing anyway.
While not as flexible as the sefton "any heat" system, the baxi appears to be very good in the scenarios for which it is designed. If you want to use a different heat source, then I guess their sterling engine can be separated from the boiler by an engineering hack, or better still someone might know who manufactures the sterling engine inside and whether it can be supplied separately.