In my personal experience, using Word for documents with more than 20 pages has not been a pleasant experience. Obviously, that could be my own bad luck, but that is also the impression I've got from other users too.
With Latex, I've never found such problems. Additionally, you are free to split up large documents into smaller chunks and then let Latex combine them altogether later (like one chapter per file). It can also create tables of content, indexes and bibliographies easily, even on multi-file projects.Stability
One of the reasons why perhaps so many people struggle with Word when creating large documents, is because it is prone to crashes. 'Document recovery' is now a high ranking feature of Word. I'm sure people would prefer if MS would just make their software more stable! (NB stability issues are not necessarily generalisable, so I'm speaking from personal experience, and of my friends and colleagues - I do not know of a single user who hasn't lost work to Word, but that's not to say that such people don't exist.)
Because Latex is so mature - and developed by extremely clever programmers - bugs are negligible. And even if it were buggy, then there is no risk of you ever losing your original source text. Where as with Word, almost any tool within its integrated environment is capable of corrupting your file if it causes a crash.
Oh, you don't need to worry about macro viruses either!Cost
Well, this is one area where Latex wins hands down, since it is free! As with most open source software, the phrase "you get what you pay for" doesn't hold true. You get an extremely mature system, that is still years ahead of its competition.What about spell checking?
It's a good point. This is not a deficiency of Latex, because it just processes the words you give it. However, within your text-editor, you do not get fancy lines highlighting your spelling errors or bad grammar as you type, like you get with Word, yet it's a feature users have come to expect when writing documents.
For starters, I do not really care for a grammar checker and anyone who actually relies on it when using Word would be better off buying a book (or looking at writing style guides) than taking the useless advice it provides.
Secondly, the 'auto-correct' feature - whilst looking like a good idea - is not beneficial in the long run. Sure, it corrects the common typos that we all make. However, the problem in my opinion is that it means we don't learn from our mistakes, e.g., you will continue to type 'teh' instead of 'the' because Word will sort it out for you. Having said that, if that's your thing, then you can easily configure any decent text editor to perform the same task. (You could, if you really wanted to, use your favourite word processor as your text editor - but then you back to square one on the stability issue.)
And so on to spelling. The great thing here is that you have a choice! Aspell and Ispell are the most popular spell checkers I know of (both open source). These will check any text file you care to feed it and you can easily configure a decent editor to integrate its functionality from within the editor itself. How to get your text editor to utilise these programs is obviously dependent on your editor of choice. Some, like Kate, interface external spell-checking programs without any effort. I personally use (g)vim which can be configured to use spell-checkers like Ispell.