co.vu is a hosting site which allows you to claim a subdomain under the co.vu domain. I have experience with this site as my tumblr blog runs under a co.vu subdomain. (My tumblr blog is leafblade.co.vu)
The only concern about co.vu is WHAT people host on it. The website itself and most of it's subdomains do not have any harmful content, but of course, just like with all domain hosting websites, some do. The hostee itself should not be rated badly for this as they have no control over the users.
This is a free domain/hosting site, commonly used for blogs and tumblr users for short, custom URLs. The site itself is quite handy, I've used it for several years to host my blogs. It's not the co.vu site itself that's bad, it's whatever potentially malicious people host on their own custom *.co.vu URLs, as with any other URL site. (ex. some URLs through bit.ly may be malicious or spammy, but not all bit.ly URLs are bad.)
Often exploited for scamming purposes, co.vu is actually a good site, and they often service those searching for a custom, free url for blogging platforms. i.e. tumblr. The site itself is not malicious, it is the scammers who use it for scamming that soil its reputation.