My understanding is that FL is based on Epic, not tabletop standard, so, you know, way bigger scale. Plus, you know, Orcs v SM is rather limiting. But I might check it out, and see what's what.
Megamek was fairly fun back when I played. The downside was that it's entirely fan-designed (and therefore dependent on the whims of the fanbase), and graphically rather dull. On the upside, it's near-completely functional, and that's what I generally feel would be missing.
I suspect a fully-functional 40k game is hindered not so much by the live-development aspect of the game, so much as such a system with strong graphics would impact their ability to sell models: the product line that is really the bread-and-butter of the franchise.
I kind of tend towards a certain level of gameplay purity. I like my 40k to be tabletop 40k. Designing a functional, or even great, game and slapping some franchising on it generally just ticks me off, for the most part (Fallout 3 being the notable exception, in my book). For me to be excited about a 40k-franchise game, it needs to essentially allow me to play 40k with friends or strangers across the map that I don't necessarily have the opportunity to hang with in-person, plus, you know, hey, not have to buy all the friggin' models (i.e. include all the army lists, not just SM v O). To get me fanboy-excited, maybe it would allow me to customize my army with assorted color and design-schemes, and allow me to zoom in for a "squad-view" to determine which targets are viable or not, I dunno. (Necromunda in this fashion would be hawt.)