The games are different enough that it could be a challenge, but it'd be easier to dumb down the PC version while in multiplayer than to sharpen the mobile or console versions. Play to the lowest common denominator. Minecraft for Windows 10 is compatible with all Minecraft Pocket Edition platforms (Windows, Android, iOS).
Ok... I know this has been a couple years...
But I do want to throw a couple things out there.
Minecraft Bedrock edition is the official version of Minecraft currently. It is cross platform between all versions including desktop (PC, Mac, Linux), mobile (Android, IOS, Windows, ect), and CONSOLE (Xbox and any others that I don't know about). Play station is not cross platform because Sony doesn't like to be cross platform for whatever reason, but they still have Minecraft on it.
Desktop has a second version, the Java version. This is where all of the modded community is. They still support/update the Java version even though it isn't the main version anymore. All bedrock versions are not compatible with the Java version.
Before Bedrock came out there were a variety of differences between the different platforms. Mobile and conaole didn't have nearly the content that desktop had. So in order to make it cross platform... they didn't dumb down the desktop version, they brought the other versions up to the desktop version. They lost the content that was unique to the mobile and/or console versions, and I'm sure they made several people mad at that, but they did it and look where it is now.
If you look at terraria, while the different versions have their differences, they aren't that far off. As you said, they ported most of the content from desktop to the other versions. However they still have their differences. If, like you said, they were to dumb down the desktop version for multiplayer only, then you would basically need to have 2 copies of the same game just to run 1 single instance. Then you run into the problem of what if two people, both on desktop want to play together, since its multiplayer, does it play the dumbed down version or does it stick with the desktop content.
I feel like it would be easier and more effective to port everything from desktop to the other versions. You might make some people mad because you got rid of the content that was unique to mobile/console, but I think you would make more people upset if you dumbed down the computer version. If you rework the entire system for mobile/console, and even some of the desktop, it might make more work for that first version, but it sets it up for easier updates later. Then you can play cross platform with out worrying about the differences because there aren't any differences. If you're worried about mobile specs not being able to handle it, don't. There are plenty of other mobile games out there that would be more graphically and resource intensive than Terraria. Also, the mobile version is surprisingly close to the desktop versions. They got rid of some content and replaced it with some other content, that's it. Theyźre already practically the same game. Sure the two versions are probably processed differently in the background, but i don't think that it would be that much different/harder processing the desktop content.
In conclusion, no, there is still not cross platform. However, it wouldn't be too difficult to make it if they decided to do that.