The problem is the Internet, not Construct. Any software using the same approach will have the same problems.
How do you figure that the problem isn't Construct when you're marketing Construct as such:
"Publish your games to just about any platform out there!"
"Create Multiplayer Games"
"Make multiplayer games with our tools and services"
"The easiest and most powerful tools right at your fingertips"
"Bring your ideas into reality. Construct's been designed to allow you to build the game you've always wanted to make."
"Make any game"
"Publish & Sell Everywhere"
"In a few easy clicks, you can publish your games to Steam, iOS, Android and a host of other places."
"Get your games in front of millions of potential players and make the next big hit!"
"Runs in the browser. Translated into 6 languages."
Only to have the Multiplayer plugin not actually work unconditionally on mobile devices in the case that a player's mobile carrier has certain NAT restrictions?
If you're marketing a tool as capable of doing X, Y and Z then the tool needs to be able to do X, Y and Z unless otherwise stated?
Neither game developers nor end-users will care what technical restrictions there are. If you're marketing X, Y and Z then you either have to deliver X, Y and Z or provide some very clear asterisks. How would you feel if you bought a game, only to have the game not work on your specific (yet common) hardware setup because of some obscure restriction, and have the developer of the game go "sorry, the engine provider's API doesn't work with [setup X] and we bought into the engine thinking that it'd work"? Because that's what I'm going to have to do with my players, and I'm not comfortable treating them like that.