I was just about to ask something similar! I get huge amounts of variation and inconsistency for yield, quality and average size, whatever method I use. For yield, I've found precipitation based methods to be more consistent. I seem to lose a lot of DNA when using column-based kits. I normally ask the Kit manufacturer and take it with a pinch of salt. If anyone has experience of other degrading effects (like shearing) I'd really like to know!
of course when you used kits to extraction DNA you'll lose a huge amount of your yields especially when using column-based kit simply because some amount of DNA will remained through the membrane of column in addition to some yields may loses during washing or other steps .
but finally almost you will get enough and acceptable amount of DNA that you needs on next Process like experience or research even diagnosis steps, on the other hand polymerase chain reaction ( PCR )technique which do the replication of DNA out of the cell will give a huge number of copies of your region in a few hours
all method have advantage and disadvantage and you can get high concentration of DNA during manual extraction but you save your time by kit.
Thank you for your input. I used FastPrep12 machine for DNA crushing. before I joined a company we do 4 cycles (4 times 45s crushing time). I checked 10 samples for 1 cycle, 2 cycles, 3 cycles, and 4 cycles to optimize and to minimize shearing of the template DNA. The two cycles produce better result. So we stick to two cycles only and produced better DNA quality as seen on NanoDrop200C. The same test may be tried if you use TissueLyser II (Qiagen)