You should use game_text for maps for Half-Life, whereas you can make use of env_message and titles.txt if your making a mod. This means you can make small tweaks if you made a typo, without needing to recompile any maps.
"All Clients" flag should only be required in multiplayer maps. I've never used it and not encountered any crashes.
The only time I did get a crash caused by an env_message was when I was actually entering the message text into the "message" field, which was too long. This was before I learned to use the codes from titles.txt
Derp...