The obvious benefit of reading a completed novel is not needing to wait for updates and knowing how many chapters you've got left.

The main reason people choose novels that are still actively being translated, is because there is more of a connection due to how you're able to see the story grow. You'll be able to chat with other fans and concoct wild theories, all whilst knowing the series could live on for many years to come. You also know that the series can develop in numerous ways since it isn't set in stone yet.

Also just the impression that an active story is newer although that isn't necessarily the case.

Everytime you finish reading an extremely long novel, you won't really feel a sense of accomplishment. You might feel like you just lost a great pastime and or even a bit empty. That is especially the case if you are a binge reader (the only type that could theoretically finish those kinds of books).

The only completed novel I ever read was Zero no Maria which I really enjoyed. Based on my memories, I'd highly recommend it. (It's actually the only book I ever recommended to my friends) It also has nothing to do with cultivation and reincarnation. A very original book.