I don't think myrtle needs good soil, but it's certainly picky. It also grows much better with summer watering than without, at least for the first few years. In our Greek garden a myrtle that was irrigated for about five years but is no longer is now a magnificent specimen, covered with fat fruit. But cuttings from it (i.e. exactly the same genetic material) which we have grown on and then planted out in the "managed wilderness" part of the garden and which have had deep monthly waterings in their first summer but are now not watered are mostly slower to get going properly - they do seem to eventually. The ones which grow best seem to be the ones which start life with a bit of shade, either from neighbouring plants or from something solid like a wall.
Having said that, none in our hot Greek garden seem to grow quite as lustily as in our Sussex garden in the UK, which is clay on sandstone.
Incidentally, our subsp. communis has exactly the off-white fruits described earlier in this thread by Cali.