I'm working at a client's premises on a project with Qlik Sense
There is a need for Alternate States usage
and we found out that there are extensions for this purpose
but the client is wondering if these extensions are reliable especially after upgrading Qlik Sense to a newer version
In my opinion, a reliable extension can only be one that you either created, or you can 100% fix it yourself. In other cases, there are always risks. Even if for today the extension is excellent and is supported by the developer.
It's just my personal opinion.
I agree with Andrey. Until Qlik certifies and supports extensions then I would not build a critical app using one. I have seen too many break or be neglected under the current system.
If you can write your own and support it then you should be ok.
I agree: while it is not likely that the alternate state functionality will break the extension might very well break in a future Qlik Sense release, even if it only uses the published APIs. Somebody needs to take responsibility for the extension, before upgrading your Qlik Sense installation you need to verify that the extension works with the new version and posibly not upgrade if it doesn't.