When a user comments, the question OP gets a notification, plus the user they are pinging, and potentially the answerer depending on where the comment is.
So this is already a potential of 3 users being pinged from one comment.
Also, how are the limitations a bad idea? They stop real problems from occurring.
Maybe the "titles" as you listed in your question don't do the "reasons" justice.
Spam control? (doesn't make any sense to me, but that's what the post
Imagine a comment:
Psst, wanna buy some viagra?? - @James, @DavidMulder, @You, @Them, @someoneelse, @bob, @sally, @foobar
This would also go to the OP and potentially the answerer too.
So this could be used for spam purposes, so not allowing it "stops potential spam".
And I take "spam" as also "unwanted" and "bothersome".
Imagine a user pinging 3, 4 or more other users to say "Thanks" or "help me".
This needs to be avoided too.
Speed? (Regex needs to match only first mention)
I'm not sure what you mean by this.
If regex was used to match only the first mention, then you still wouldn't have multiple notifications.
In terms of being able to work around the limitations, and use @ for multiple users and only ping one, see the bottom of my answer.
The "speed" issue will likely take into account the fact there is constantly a lot of things occurring around the sites - questions, answers, comments, reviews, flags, votes, etc.
Even comment notifications alone:
There are 4 million users on Stack Overflow, there will be many notifications sent every minute.
All needing scripts: When sent; Save in DB; Show to pinged user(s); DB updated when user(s) read them; etc.
With everything else going on around the site, having bulky comment notifications is not a good idea.
Comments are not that important anyway, certainly not for pinging multiple users.
It's been debated to death that Stack is not social network sites, so comments and certainly pings are supposed to be a means to discuss necessary things to obtain the answer, not chat, or debate etc.
Technical limitations? (Comment on question: System can't handle
sending multiple notifications...)
Not being a dev in Stack I cannot state why, but I imagine that there will be some truth in this.
And even though they might be able to work around the technical issues, considering the spam potential, speed issue, and other things, it just won't be worthwhile.
If you are having a discussion with two or more users in comments other than the question OP and answerer, then they'll likely be checking back on comments anyway.
If not, and it's very useful or important, then either start a temporary chat room between you all, or in rare and very useful/important situations just use two comments to ping both users.
You stated "Regex needs to match only first mention".
This can already be worked around by the users anyway, by using the backtick.
How do comment @replies work?
Comments containing more than one @name are blocked unless they
contain a backtick `. In the latter case, only the first name
mentioned using the @name syntax will be notified. For example, @alice
`@bob Hi!` will notify Alice (if she has participated in that post),
but not Bob.
An exception is the case when the first @name either matched nobody,
or matched the post's author (and thus isn't necessary); in this case,
the next @name will be checked.