It's going to work just fine, the email integration (for your paypal products) will get triggered.
As for downtime:
The app is safe to use, and it behaves just like a Paypal button, in worst scenario if it goes down your funnel will still keep working just fine, the issue that has happened earlier was just a very temporary "mistake" and it got resolved on time.
For that please use the "Translator" under "Tools".
Our own order summary, and many more functionality (to improve the looks of your funnel) is integrated within our "Fancify" app, feel free to test it out. in case you are some 3rd party scripts that interfere with that preferably remove it.
Generally, it shouldn't be interfering with any code you have, unless both our apps and the custom codes you're using are both trying to "modify" same areas/sections of your funnel step.
Hope that helps!