I'm wondering about the reliability of the feature that allows to
address other users in comments using the @user syntax.
"Wondering" means you don't really know enough about it to propose it's wrong or could be improved.
You really should do some reading and once you know more about the system you will be able to identify problems and think of possible improvements :)
Here's a good starting point:
How do comment @replies work?
The Stack sites are made by people with experience, and tried and tested functionality in place.
User names can be changed. Because they are not unique the system
isn't able to display the comments properly after a change since the
system doesn't know which user was addressed once there are more users
with that name. Result: The @user expression keeps unchanged, the name
of the addressed user is now wrong.
Yes this is true, you have identified a real potential issue, but it's not really a major or frequent issue.
The Questions and Answers are the most important things, comments are "useful" but not essential.
And going as far as being able to ping someone with @user in comments is nothing more than a nice feature to have, which is also "useful". They are not "essential" however.
I'm really not sure what else could be done, other than updating a HUGE load of databases/tables to update their names in comments. This would seem like a mammoth task for little gain. Certainly not worthwhile just so some of us can occasionally still ping their old comments.
Considering there are 4 million users on Stack Overflow (for example). That would mean scripts doing lots of searching through many DBs and tables to find all their comments in all questions they commented in, and then many DB updates to change their name. For all users who change their username.
This is very likely to have a detrimental effect on things such as site speed due to resources being used, and tables being locked.
Lastly, comments can be and often are cleaned up/deleted.
So changing usernames in comments is not really viable, or worthwhile.
Can you suggest a reason why it's a serious issue not being able to ping someone in comments because they have changed their name?
You can always ping the questioner and answerers, which is the important thing. Anything else is never going to be "important" to the site or the Q&A itself, and is always only "useful".
Are there any plans to implement that feature in a reliable way? Or
don't you care about that because you think it isn't a major problem?
Most of what you raised isn't a problem at all. And/or there is probably no better ways to do it.
The site and users are open to suggestions, but you can't just jump on pointing out flaws in things you have not tried, tested, and understand fully. That's not fair really - and is probably the main reason for your downvotes.
You point out mistakes and problems where there is none, and suggest the site doesn't care, which is very inaccurate and again is an unfair "presumption" to make.
Have a read around the site about how comment notifications work, and the other suggestions people have made in an attempt to improve them.
After that, if you then have a query or good idea, don't hesitate to suggest/ask :)
Prior to the question edit:
if I have a conversation with Jon Doe on Stackoverflow and address him
in a comment, multiple users might get noticed.
Again you state "might". How can you contest something you do not know about?
It's almost like you are saying "I do not know how this works, but if it does X when we do Y then that would be a problem".
The pings don't ping anyone/anywhere on the site. Otherwise that would just be silly.
It's only in the same thread where this happens.
See the link above, e.g.:
"Matching is performed in reverse chronological order, so if five people named John are participating, @john will match the most recent John. (Use the next rule to differentiate.)"
I don't see how else it could be done (without magic).