OVH frequently has problems with false positives, or so some of their customers report. They may offer good value for non-critical services, but they aren't exactly known for 100% uptime.
I've been using them since March, and have had 100% uptime during that period. I haven't had any false positives, and the one time they did mitigate a DDoS I didn't even notice it. As I understand it you'll just have a slightly higher ping time when the DDoS is active. An occasional false positive with slightly higher ping time seems a better option than your site being guaranteed to get null-routed for 24 hours (as happened to me on iweb, although I managed to convince them to remove the null route after a few hours).