If the usage terms say "do not use in any critical applications", I would've thought that the responsibility for using the code in that fashion woudl be squarely with the entity that did the integration?
It'd probably be better if the usage terms would by necessity spell out that you were happy for the code to be used in life-critical situations, instead of having to opt out of it.
The default usage terms are "all rights reserved, nobody can use this but me". You change this by applying licenses which regulate the terms under which you're happy for the code to be used by others. The vast majority of popular Free Software licenses allow you to use the code under no guarantees whatsoever, so if you want to use some software in critical applications and hold its authors responsible if it doesn't work as advertised, you should probably pay them and include this responsibility in their contract.
It'd probably be better if the usage terms would by necessity spell out that you were happy for the code to be used in life-critical situations, instead of having to opt out of it.