I did some more detailed tests, and got some interesting conclusions.
In brief, when flash output power is high enough, or it is placed close enough to flashmeter, as to get apertures from f8 upwards, Lunasix F readings lead to nice exposures, very close to that of the dedicated flashmeter.
Troubles are when a good exposure requires apertures f5.6 and below.
Then, Lunasix F overexposes by 1 or 2 stops (reads f2.8 when f4 is the good one), and leads to (almost or sometimes completely) unrecoverable images.
So, the meters work perfectly, except when reading flash in incident mode, and flash requires apertures around or below f5.6.
Does it makes sense?