I've heard that feminized seeds often turn out herm. and that yield is decreased. Can anybody shed light on these rumors for me?
Feminised seeds are produced from a genetic female plant that has had one or more of it's branches treated with hormones, this forces the plant to produce some male flowers. The pollen is used from these male flowers to pollinate the rest of the female flowers thus producing seeds that do not contain a male chromosome. Whilst that means they cannot be male, it also means that they carry a hermaphrodite chromosome alongside the female one, so they can be female or genetic hermaphrodites.
Fortunately, and this is where feminised seeds become marketable, the ratio of females to genetic hermpahrodites is much larger than for standard seeds ratio of males to females. The feminised seeds on average should produce a 0-20% hermaphrodite to female ratio wheras the standard seeds male to female ratio is 50%.
That also means that by growing feminised seeds you have up to a 20% chance out of 10 seeds to get a genetic hermphrodite as well as the same risk of standard seeds of plants turning hermaphrodite via environmental influences.
The bottom line therefore is, that you have a much greater chance of getting hermaphrodites with feminised seeds than standard seeds but you're also virtually guaranteed some pure genetic females if you grow out enough seeds which you aren't with standard seeds. This simply means that if you go down the feminised route, keep a very watchful eye out for any hermaphrodite plants that if not spotted can pollinate all your other females.
The reason that yield is decreased is because in plants that hermie, some of their yield is taken up with male flowers and if allowed to heavily pollinate the other females their energy will switch to seed making and not bud and flower making. Lightly pollinated plants simply carry on making flowers and lose little if any yield because of it.