On the other hand, this extension is used on (select) WMF wikis, such as Wikimedia Commons, so it's code-reviewed and guaranteed to be compatible with recent versions of MW.
However, we have a very similar extension -- HAWelcome -- already, which supports SocialProfile, which NewUserMessage doesn't. (See also T68)
Also it should be noted that NewUserMessage adds a message to the user's talk page right after the account has been created, whereas HAWelcome welcomes users after their first edit. So while HAWelcome has the proven potential to create a mess (welcoming spambots), NewUserMessage has even more potential for that IMO.
I was originally thinking that we shouldn't add NewUserMessage just yet when HAWelcome is already there and available for users on request, but we definitely should look into merging the useful parts of HAWelcome into NewUserMessage so that there'd be one stable, FOSS, somewhat maintained solution for automatically welcoming users, but having realized a rather crucial fundamental difference between these two -- welcoming users right after account creation (NewUserMessage) vs. welcoming them after their first edit (HAWelcome) -- I'm not sure if that's even possible. Still something that we should look into.
Well, just in case if this means anything, Echo supports a notification for edits made for the very first time. It doesn't require us having to manually delete it on our side. The message is located at notification-header-thank-you-1-edit, so it's technically customisable. Here's a screenshot:
Obviously the difference is that NewUserMessage is much more customizable, where you can use multiple templates to create a welcome message, and probably more welcoming to a person than a message like this above. However, notification-header-thank-you-1-edit could be edited to point to an on-wiki page like Project:Community Portal or Project:Welcome (depending on whatever the wiki wants). Of course with the nature of MediaWiki, the fact that the echo message is editable probably isn't very well known by many, so we'd need to add a Help page on our ShoutWiki Hub about this (plus how the general extension works), except at something like Help:Notifications since the brand name isn't important, but the actual functionality is (plus, we need to keep the technical jargon out).