ORIGINAL: RCKen
You've already covered most of the factors. The biggie is that they drain your battery faster than analog servos. For this very reason I wouldn't use them in a trainer. And besides, they are overkill for a trainer. If you can get a good deal on them buy them and save them for your 2nd or 3rd plane.
Ken
Thanks for the reply Ken. I'm a little confused on the battery drain though. My understanding is the battery would drain faster due to the slop in the contol rods or binding.
This is from futaba.
Q: What can I do to ensure that my digital servos offer long life?
A: Your number one concern should be that linkages are tight, with absolutely no friction or stiffness from binding. Standard servos don't notice common problems like slop at neutral, but a digital servo will - and in the process of struggling to hold the rod in position, it will quickly drain your batteries and eventually ruin its motor. Digital servos always work to hold positions at full strength. If at any point the servo must "fight" the linkage to locate and hold its position, it will literally burn itself out.