First, it's nearly impossible to achieve the same riding position for a road bike and a mountain bike. You can get close, but you'd probably be making serious compromises either way. The way I see it, road bike geometry prioritizes comfort and power delivery while seated over long distances, whereas with MTB geometry, the priority is handling and stability on rough terrain. It is likely this handling and stability that makes you feel more comfortable going all out on the MTB frame. Downhill and freeride bikes have head angles that are so slack, that when coupled with plush, long suspension travel, some designs can barely be pedaled efficiently at all, but they are astonishingly awesome for their intended purpose (drops and steep rollers that would have lesser bikes sending you over the bars every time). A cross-country hardtail is a somewhat of a compromise between these designs.