Wikis like Wikipedia get a lot of flack for their openness to anyone to edit and their therefore perceived lack of quality. Wikipedia articles typically do one thing though that really puts it above a lot of other casual sources of information on the internet- information sources for each article are cited. You don't see that too often elsewhere on the internet when you're looking at anything other than scholarly articles.
We tell our students that while wikipedia isn't acceptable as a cited source on a final product submitted by the students, but that it's a great place to start gathering information. A wikipedia article can usually give them a decent understanding about a topic of they know little about it to begin with, and the citations can usually point them in the direction of at least a couple of acceptable sources of information that they can use in their work.
Certainly, with any wiki, the quality of information is going to be an issue. Typically, though, you see that when a wiki has a dedicated community, it usually polices itself pretty well. Willful and intentional posting of incorrect information is usually caught and dealt with pretty quickly.
Unintentional posting of incorrect information is a little bit harder to deal with, since it's not as obvious. I would think, though, that a hiking wiki would be read mainly by those with less hiking experience, who are more likely to hike on more popular trails. Since popular trails get hiked more often, I would think that the bulk of the information being accessed on the site by the public would concern these trails, which means that this information would get "field checked" pretty regularly.
I also don't think that it would take much effort at all to provide information that is better in quality and more up to date than the standard for backcountry hiking information- the guidebooks. The fact is, when you pay close attention, many hiking guidebooks are full of missinformation. None of it is willful, of course, but when you have a guidebook that is written by one or two people, and covers an area big enough that it would take months to rehike and recheck all of the trails, there are going to be inaccuracies that end up in the final product.
Because a wiki is much more flexible, and bits and pieces of the overall package can be updated as people hike those trails, the overall quality of the information offered could be quite high with a dedicated community. I think that, in terms of the overall quality of the information on the site, the flexibility of the wiki overall to remain more up to date would more than offset the any inaccurate information on a specific level. In other words, yes, you might have wrong information posted to an individual article, but overall, the quality of the information would be among the most up to date and best available.
As for writing style- yes, it could be difficult to achieve a standard that is non-subjective. Plenty of other wikis have accomplished this, though, and with the overall intelligence level of the online hiking community, I don't really see it as a huge stumbling block for a project like this.