Why Typing Hero doesn't work with X app
X app is most likely using a custom text input component. You can see a list of apps that are known using such components at Limitation page.
Why Typing Hero stops working after a while
It is highly like that Typing Hero Service (a background process) is killed by Android system. For Typing Hero to work properly, it must run in background all the time.
Some device manufacturers aggresively kill background processes in favor of battery life by running battery optimization or battery saver apps. To solve this issue, please follow the instructions on Don't Kill My App specific for your device manufacturer.
Is it safe to allow Typing Hero to run in background all the time
Yes, I swear by this! Typing Hero is lean and efficient. I'm not building an app that will drain battery on my phone, it just doesn't make sense.
If you have doubt, I recommend you to try enabling it for a few days. Regularly check if it shows up in the list of apps that's draining your battery. If it shows up, please uninstall it and inform me.
Why the keyword is not replaced after typing it
Make sure that you did not check “Replace keyword with text after typing space” when configuring the snippet. If that option is checked, it will be replaced after you type the keyword followed with space.