Some niches are so saturated with sites, its difficult to compete. But on the other hand, a lot of those sites do not offer any real quality information.
I dont know which one is worse:
1 - competing against a company site that spends thousands of dollars on ads.
2 - competing against a spammer site that google ranks well for some reason.
I have seen this lots of times - you build a good quality site, with lots of good information, and the search engines barely rank you. Someone builds a trash site, and they are ranked at the top of the list - go figure that one out.
One thing is for sure, if your going to build a long term project website, you better have plenty of time, and plenty of patience. Either that, or have plenty of money to buy ads.