For a single supply it is the same as a bridge rectifier when the single load is taken from plus to minus. May be an advantage in cases where dual supplies are needed but otherwise it's just a 3 phase bridge and the neutral is irrelevant.

Now the transformer bit.
I feel certain that the transformers in the inverters you are referring to are high frequency ferrite devices. In general the inverters using power frequency transformers have lower efficiency.

I suppose within reason a transformer can have as high an efficiency as you want if you make it big enough and costly enough. You can reduce copper loss to negligible proportions by using thick enough wire. You can't entirely loose the iron loss, but by keeping flux density low ( lots of turns/volt) and using the highest grades of "iron" possible you may be able to get a low power transformer up to low 90s%

Small electrical machines are inherently larger in proportion and less efficient than their monster power station brothers. I think Joestue hit it exactly with the cooling bit. To be practical and cost effective for most applications the thing needs to be efficient enough to survive its temperature rise. A 1kVA transformer may be able to get rid of 15% loss easily enough. Trying to dissipate 15% of 50mVA is a very different problem. The cooling is a major issue for units running at over 98%.

Typical small transformers are probably considerably worse than many imagine with many cheap units coming in at below 80%. The average customer will not pay the price or accept something weighing 5 times as much to gain a few %.

The other factor is frequency, the lower the frequency the larger it becomes for a decent efficiency, but above about 250Hz the choice of iron becomes very critical. Many of the aircraft transformers are a fraction of the weight of the 60Hz things but the efficiency may be little better as once again they are optimised to be most cost effective for the job.