Dimmable ballast cons ?

I am thinking of getting 600W dimmable light system on the top of my 400W HID for expansion and flexibility.

Any issues like light spectrum change, bulb/ballast lifetime or else with "dimmables" ?

Please do not try to convince me to CFL's (been there) neither I am going to blow my money for expensive LEDS cause any current models get obsolete in no time, since new "improved" versions are coming on the market like a clock..

I've been using them for years with none failing. I never use the dimmable feature on the 600 I think it is more useful on a 1000. As long as a clone has roots it will handle a 600 without dimming.___ More light faster growth. Big bag of dirt big bag of buds ___