Assuming any relation to past experiences looking at software code bases. I worked at a software company doing support/code maintenance on proprietary code layers of which most of the low-level layer was developed in the 80s, built on a language/database that was created in the late 70s(iirc). Most of the middle layer was done in the 90s using a proprietary language and is still being updated til this day(where I spent most of my days), in the late 90s the mid-layer was modified to run on 2 different SQL DBs while still being backwards compatible with the old one, the top-most app layer was made in 2010 on one framework, 2011 on another, then again in 2014 or so on another. Not to mention it's ties into other a myriad of other things that's all contained within a singular package, despite a half dozen software languages being used in that single package, then we had to integrate it with other internal and external software. Man... good times! I'm totally not balding just thinking about it!
Mugen is smaller, made by a much smaller and more focused team, has no net code, doesn't need to integrate with RPGMaker and Minecraft at the same time, but is still probably a beast of burden!
Always expect the worst, because if you don't you'll go insane and if you're wrong you get pleasantly surprised! I love it when I'm wrong, which is never, hence why I hate everything.