I don't really know for either of those. I mean, I could at least take an educated guess, but I've never used Metagross in OU and I've only used Uxie once in UU (and it was a scarf variant). Some reasons for choosing a specific lead, though, isn't just based around what it can do as a lead, but what else it can do for your team as well. It's not as black and white as I tried to make it.
Metagross has the ability to tank hits from leads, set up SR if not Taunted, take out leads that do Taunt it, and still come in later to Explode on something your team doesn't like much (or set up SR if that's more important). I'd basically classify Lead Metagross as offensive support, if that makes sense.
Uxie's use as a lead is kinda complicated to explain, but basically it revolves around its ability to have something good to do in all situations. If they can Taunt, you U-Turn. It's not going to matter that you switched out anyway, because Uxie's so bulky that it can basically come in whenever it wants later and do what it was supposed to do at the start. I'd probably use Uxie as a lead if my team just simply needed team support, and I couldn't find a place somewhere else. And seeing as that tends to happen for teams quite often, it only makes sense that it finds its place among the top UU leads.
I may be wrong, but those are my theories for their uses as leads. Now obviously a Pokemon like Uxie can't necessarily "lose" in the lead position, but at the same time it can still "lose" if it doesn't get a chance to do the main job your team needs some time during the match. That's the catch to using something like that.