I've now watched both of the episodes that have aired so far and I must say that I'm enjoying this show, I really am. But there has been literally zero reason for this to be X-Men related show. All you would have to do is change the main character's name and remove all references to these powered people being mutants and you would hardly even realize that this show has any kind of connection to X-Men franchise (any version of it). I know it's still early and they might reveal more in the upcoming episodes which I am truly excited to see regardless. It's just the x-fan in me being disappointed that we finally get an X-Men related show and it doesn't have anything to do with X-Men (and from the way it is going it may not have in the future either).
And I'm not saying that I'm expecting any big X-Men characters to appear. I'm just fine with them focusing on one c-list x-character and most of the supporting characters being completely original characters. But from what I can tell David's character and his background have been changed so much from what it is in the comics that it doesn't really bear much resemblance to him. Only thing these 2 have in common is that they are powerful mutants struggling with powers so great that it has unhinged their minds. But that's pretty much it.
To me that isn't reason enough to call this show Legion or call this character David Haller. But let's wait and see what future beholds for this show! For all we know at this point they might reveal some more concrete connections to x-verse in the upcoming episodes! And even if they don't (I wouldn't be surprised by it), well, I'll just enjoy this show for what it is. And in any case, it will scratch my itch for shows about superpowered characters (who are struggling with their powers while running from shadowy government forces).