Couple of things in addition to what others have said:
I'm not sure that climate is a HUGE deal with these plants overall (depending on cultivar) so much as size/ age and cross-pollination for some varieties. We have some local hobby growers here in the desert who have managed to get a fair amount of decent-tasting fruit set in our less-than-ideal climactic conditions of 112+ summers and 35 winters (
https://www.youtube.com/watch?v=T2XasFW8BaQ is an example; out here it's managing the sunlight against the heat/burn since they need a lot of light to flower here). And New Zealand climate where they primarily grow them commercially actually varies quite a bit (
https://commercial.waimeanurseries.co.nz/assets/Uploads/Comm-Feijoas/WAIMEA-Feijoa-Brochure.pdf) but relatively warm, ranging from subtropical to temperate. Northland, Auckland and the Bay of Plenty don't get much below 45F most of their winter, rarely freezing although can get cold. Fwiw the plant's origins are in southern Brazil/ Argentina similar to other edible myrtles like jaboticaba but higher elevation iirc.
Seems like, climate-wise, they do well where they can get a lot of sun and not burn (in the states, the best tasting fruit I've had was from trees in or near Portland fwiw). Size and age might have more to do with it too, as well as cross-pollination. Personal experience is that even the "self-pollinating" varieties do a lot better with a cross-pollinator. Like Nate said, your trees are pretty small to be bearing much fruit, and as you say the Nikita may not be flowering at the same time as the others. The taste might be down to they just don't have enough age yet to be making decent fruit.
I'd agree with MisterPlantee about trying an airpot/root trainer if you have to keep them small, to at least get as big/healthy of a root mass as possible if you have to keep them small on the top (those pots also make root pruning a lot easier ime).
someone did a study and feijoa sweetness improves with how low a temperature they have, they tried feijoas from zone 9 10 11 12 and pretty unanimously decided zone 9 was the best tasting, hear that zone pushers have even better tasting fruit; heat will make the fruit drop/not set
Makes sense, lots of plants produce extra sugars when it gets cold to guard against cellular damage, that's why broccoli tastes better in the winter. NZ growers info says that the fruit can get damaged in the winter though; guessing this might depend on cultivar.
We do get fruit set here in the heatstroke helllands though, when the tree gets big enough.