I am always in favor of more monster races. It's pretty much all I play unless I don't have access to them at all.
As far as NPC reactions the Yuan-Ti Pureblood would actually be very easy to do, I think. As has already been noted the ENTIRE POINT of the Pureblood caste is that they can pass for human with minimal effort and are frequently used as infiltrators and spies. If normal NPCs could tell your Pureblood a part from say, a Dragonblood Sorcerer or just a human with weird eyes then the entire role purebloods serve within their society wouldn't be viable.
That means NPCs could and should just react to your Yuan-Ti like they would looking at a human. You could then get some dialogue where you reveal your true nature where it is advantageous to do so, say to boost an intimidation check or when trying to join the Absolute or something. That way only select NPCs need a hard reaction to the revelation rather than literally every NPC with eyes.
Heck, maybe make it something about your PC that the companions actually react to with surprise. Why should they be the only ones that have a shocking secret?
In a similar vein, it'd be funny if as a Lizardfolk you had the option to lie to people about being a Dragonborn. After all, how many commoners are going to know the difference between one large, reptilian humanoid and another at a glance?
