Mainly performance related issues, we were on the milestone backend which would constantly freeze up. We never had a successful upgrade... ever. We felt like making a change to the system or recording parameters was like playing Jenga, you weren't sure if the system was going to crash and burn, or actually make the change. Every time we ever called into tech support, they always answered "never heard of that happening before". We are at the point where we stopped selling Onssi unless we absolutely had to, and we are not leading new projects with it. And the whole change over process from Milestone to the new recording engine has just been a nightmare, having to basically set up the system from scratch... ok on a 700+ camera system that is a freaking weeks worth of labor resetting motion detection windows and camera names, then going back through each camera to check the settings, finding out you missed important events because your settings were too light... having to find that perfect balance X 700+ gets old very fast, especially since you just had to do it the last time the Onssi system crashed and burned from the last upgrade which somehow corrupted the SQL database beyond recognition.
System is configured properly, according to tech support, hardware and storage exceed what ONSSI presales engineers recommended and still the performance is horrible. So yeah we don't lead with Onssi, been burned way too many times.