Another concern I have is for quality, in addition to the very valid concerns abyssalmonkey has. The main barrier to entry is setting up a website and uploading stuff, though there are some certain sites that allow you to upload there and get paid, or something. If you want to translate a novel, basically, you probably need to make a wordpress or something more. It takes some commitment. With manga, there is definitely already an issue with snipes that are considerably low quality, but this can be much worse with novels. There are plenty of MTL trash out there, and I think a site like mangadex for novels would exacerbate this problem, making it easier for people to upload their shitty MTL. With manga, you have to at the least, spend around 30 minutes shitting text onto the page with editing, and OCR + google translate to make a translation. It doesn't necessitate much effort, but it's something. With a web novel, you can just copy paste it into google translate, and while MangaDex claims not to police the quality of the manga uploaded(only troll chs removed), this would definitely need to be addressed in a novel agg. The line between MTL and translation is already blurred with some of the shit out there, and MD would have to decide whether a translation is edited enough not to qualify as MTL. And if you have to make such choices, the quality is already gonna be p shit.
Another thing, NovelUpdates has a sniping policy that IMO, should be followed by this such potential novel agg. Otherwise, it could be disruptive if MD comes and makes sniping easier. The NU policy is, in short, 3 months without an update until a different group can submit a translation. But, you can still submit a competing translation if you also start at ch. 1. MD as a novel agg could give rise to sniping in ln/wn translations if such things are not dealt with, and IMO they should be.
tl;dr: NovelDex would make MTL more widespread and further degrade the quality of novels overall in addition to sniping lns becoming a thing, if it's regulated the same way MangaDex is.