Just that some things don't work as you might expect and some things don't work at all (looking at particular files and file types) - I'm sure I read there were restrictions but I have gone back for a second look and can't find it now!
There are certainly restrictions on how you restore partitions etc. and you are restricted by having to create partitions at least as big as the one you backed up (even though the partition may only be 10% full) which is a little irritataing and could potentially cause problems if you have had to alter partition sizes and then retrieve an older backup.
You also have to download BartPE and create a bootable rescue disc tailored with this app. in order to restore images without a working copy of windows (or to restroe your windows partition) ... don't forget to get this sorted out in advance and check it works. BartPE is a great tool but sometimes the plugins for it can be a little flaky!