i experimented with that stuff a while back, the installer files had read me doc's included that I read and they answered all my questions.
boot screens change system files and do have a small element of risk, you should backup those files first and read me doc's should provide instructions.
window blinds runs in the background and doesn't replace system files, very secure, but there is a performance hit since it runs in the background.
I've used this some too themepacker, has all kinds of themes, like bootscreens it replaces system files so there is an element of risk, but this doesn't run in the background eating resources.
So the bad on window blinds is that it makes your computer a little slower.
The bad on bootscreens & themepackers is since they replace system files, if something goes wrong then it could be bad, the risk is supposed to be a small one but window blinds doesn't carry this risk.