This week's reason for hating Decklink cards! - I have just got back from a client who was complaining about colour variation between component and SDi outputs of a Decklink Pro card. No trouble I thought - align the monitor so I'm happy that the reason is upstream and attack the ProcAmp settings. When you get there you can only tweak the U & V components together and they don't track!
So, I put up a PLUGE signal (so no colour, only luminance), cranked up the gain on the (digital) vectorscope and looked at the colour-in-the-noise (as if you were doing a quick colour balance on a studio camera looking at a chip chart). Tweak the chroma gain on the G5 and the colour caste shifts!. I hate those cards - they are so domestic! Now - the DeckLink defenders will say "but you can unlock the colour components and tweak them separately" - try it - one effects the other terribly - as with their analogue and digital stages they have used some half-arsed implementation that goes to prove that even in 2005 $895 doesn't buy you a broadcast-capable interface card.
So, you can really only use the analogue component output if you don't care about the colour balance (and presumably if you don't want to lay back to analogue tape) - and don't even think about using it at the same time as the SDi.

Catch up with previous reasons why I hate them here and here.