Lately I have been running into time issues with assembling miniatures, getting good looking one-piece minis would be greatly appreciated.
One-piece run into the problem of being VERY limited in pose and detail. Technology has definitely progressed so that detail is much better than the
3rd edition plastics, but the issues are still there. Different plastic will hold detail better but I haven't seen anyone get around negative spaces.
ANYTHING with deep (front to back) torsos like half the original Clan omnis (Mad Cat, Masakari, Daishi, etc.) will not come out well at all. Could be the reason we didn't see Clan star packs. That is a whole lot of the most popular 'Mechs. It's likely that, aside from the Reseen Tbolt, those are the ones most complained about multi-piece minis.
They did experiment with the Mad Cat and Thor plastics, and they were very well done. They were not one-piece, though. If you bought a pair, one came preassembled and one did not. That means they have to pay someone to preassemble. People like the plastics because they are cheaper than metal. Preassembly is going to negate this.