Broad site or niche specific site?

Im about to start a new project (long-term), however im contemplating on whether to build a site just based on 1 category e.g. fitness OR a site which has multiple different categories e.g. fitness, lifestyle, finance, motivation, dating, etc.

Im thinking of building a broad-based site, why (PROS)?

- Cost-effective (don't need to build a brand new site for each category)

- My ability to find new keywords to target expands (if i just build a site on fitness, then im stuck with just fitness keywords)

- I can write about things which i enjoy (e.g. motivation, learning) while hire writers for other categories like fitness.

- The site will build good authority overtime since i will be constantly working on it (building backlinks to new posts), and then naturally overtime posts should naturally rank without much backlink push.


Negatives
- Not really good way to diversify if it gets bitch slapped by google (but i do have other sites, and mini sites) Not too worried about this, since the site will have really high quality articles. Low chance of it getting spanked, and if it does it would be for a short-period of time because its a mistake on their end.

- I've read the Master Plan and because my site is going to be broad its not going to be ALL relevant OR themed properly, not sure if this is a negative or how to deal with it.

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What do you guys think.?

Do you think there will be any negatives from a SEO point-view e.g. like having multiple unrelated categories? Rather then a related categories. Should i even worry about this?


Examples of broad-based sites
Men's Fitness - Sports, Fitness, Health, Nutrition, Style and Sex
Men's Online Magazine | Impulse Magazine
http://www.lifehack.org/
Personal Development - Think Simple Now
Personal Growth and Effectiveness at Life Optimizer