From a development standpoint, the major issue with this approach is the opaque (not visible) dependencies between the software components on the various platforms and operating systems that make up your application (see # 5)

If a database definition changes, how do we know which web application uses this table? Similarly, if we change an ILE module, which programs call this module? In itself, this needs careful investigation and documentation which costs time (see #5).

But when it is time to go live, we must have assurance that all the tiers will go together. And if one platform fails to install, we must be prepared to take counter measurements. One of the options could be to roll back on all platforms.

Automation of this process is very important for the continuation of your business processes. You need to find a tool that is able to deploy all changes to all platforms simultaneously. 

Have you found one already?