I also lost some potted plants in a heat spell. I think the root system of Ugni cannot handle high temperatures or a sudden and large increase in temperature. The plants in the ground or in larger pots did fine.
There is some buzz about the Ka-pow cultivar, but it is not sold outside of the UK, so no personal experience. I have also heard of a German cultivar called 'Elite' that is supposed to produce a good yield. Haven't found that one yet either.
I have bought different plants labeled as "Ugni Molinae" in the Netherlands and I have grown some from seed I got from the US. Last year I bought two plants in France, one Ugni Molinae and one Ugni Myricoides. The last two plants from France look very different from each other, but also quite different of all the other Ugni Molinae's I had grown before. All those now look to me like an intermediate form between the two French plants.
So it seems to me that either Ugni Molinae can grow very different leaves in different growing conditions, or somewhere along the line the two species got crossed and what is sold as Ugni Molinae in some places actually has genetics of both.
Here are some pictures of the two French plants that are most distinct from each other:
Ugni Molinae at the top, Ugni Myricoides at the bottom:
Close-up with flower buds forming on Ugni Molinae:
And a close-up of Ugni Myricoides: