Having developed with Unity before, I feel like its security sandbox is no more or less prone to developer abuse than Flash has been throughout the years. Details here: http://docs.unity3d.com/Documentation/Manual/SecuritySandbox.html
Unfortunately, some antivirus manufacturers currently report a false positive when the Unity Web Player tries to load, and browser penetration is currently much lower than Flash.
Having said that, how did you even get Newgrounds to host a .unity file? Some sort of flash frontend around an HTML wrapper around the Unity Webplayer? While I feel that Unity is a superior plugin for web-based 3D games, circumventing Tom's decision not to host Unity files here feels vaguely subversive. Whatever Tom's reasoning, he's chosen not to host unity files while a bunch of other web game portals choose to allow them. Circumventing that policy feels like a dick move.
In any case, I couldn't get the Unity webplayer to load in Firefox with NoScript, because I don't know what domain name is hosting the Unity webplayer file. It's not exposed in the HTML, so NoScript doesn't offer me the option to whitelist it.
Bottom line: Content did not load. Zero stars.