Duh, theres no downsides in to it. As long as you go with a custom made cms
system, your directory structure can be done exactly how you like, without any mod rewrite thingies and the same goes with everything else you are worried about.
Look at my site. I build the cms system to that site myself with php. All the articles i post to my site go through the cms system. All the games, bonuses, top10 lists, menus, everything are done by the cms system. All i need to do is open the admin panel and choose what i want to add and where. It’s very simple, easy and automatic for the user. My system is not near to perfection, but it does it all. Every one of my pages is dynamic. For example, the points you can see in the top10 casinos are calculated by the cms system from various things like amount of games the casino have, support (flash,live,phone,email,us), bonuses (how many, how often, wager limits)…..
However, building a cms system as large as mine takes some time and effort. I built mine in 3 months, but im lazy. It could be done in 1. Also the price tag that comes along with the custom cms system might be a little bit frightening for some people, but i assure you it will pay you back. Plus i built mine from the scratch, so another one with similar and possibly with less functions could be done in weeks or days 
There are many other positive things in these cms systems you might not see without taking a closer look, e.g., everytime i publish an article, game or review i have an option to add that to my RSS and even better, to ping the social bookmarking services to send their robots to check me out. I can also tag all of my articles for services like technorati.
There is so much more in to this, but lets just say its like having a blog with the SEO advantages of a static website.
After all of this hype, this might sound like a sales speak. Well it kind of is, it’s just that i dont know if i myself am willing to do this for anyone. Plus the price tag needs to be right, since im no indian fela and everything is expensive here. 
Let’s just say you could find some interesting details you could study more from this post and its all for free.
I think i got more or less carried away from the subject with this post. sorry for that.
P.M me if you wish to know more about my system or the concept of CMS.