Most off the shelf CMS systems are cute for what they were actually designed to do -- like the ones that do blogs, or the ones meant to be a shopping solution.
The further you get from the original purpose of the system, like using Wordpress as a general purpose website, the less effective the result. While you CAN go that route, most off the shelf systems are garbage to begin with, vomiting up garbage HTML often not letting you remove the garbage -- generally speaking making it do something it was never meant to is just the road to failure.
A custom solution is... well. It's like going to a tailor. You buy off the rack when you're not a 'normal' size, then try to have it adjusted, it still looks like crap even if you can make it fit. (try finding mens slacks with a 28" inseam in a 40" waist -- you trim down a 32 and it looks like bell bottoms). The tailored is going to cost you more, but it's going to fit, look good, and all around just be a better product.
It is much more of an investment, but if you want to build something unique and successful, what would make you think you could just cookie-cutter out an off the shelf answer any better than any of the hundreds if not thousands of other people doing the exact same thing?
It's a hefty part of why most Internet startups either fold in the first year, or just sit there as a "also ran" money-pit.
You also have to try to avoid the "I want it now, who cares what it costs me later" attitude that pervades just about everything these days. Sleazy shortcuts and cutting corners at the start is NOT a sound battle-plan; be it from garbage "frameworks" that are allegedly easier and faster and aren't -- to not spending the time to tack down every corner BEFORE launching -- it's just asking for a project to never reach it's full potential.
Think of it like the cancer on todays society known as credit -- paying more later for something you can't afford now is NOT the road to success; and is quite obviously a giant scam. Same goes for things like insurance -- If you did it on the street, it's illegal, you do it as a banker, and the Government makes participation mandatory. Then everyone stands around wondering why the economy is in the tank. ...and before anyone out there kneejerks into "what's that got to do with it" -- it's called a simile. Literacy, TRY IT!
Same goes for building a website. You plan at a loss, you're gonna lose. The whole current attitude of "oh we can just deal with it later" and "we're ***'ed in the future, but who cares" is so massively stupid, and trying to take shortcuts like an off the shelf CMS is just another aspect of this.
See all the sleazy scam artists who rubber-stamp out turdpress installations leaving the poor site owner holding the bag with a money-pit that usually goes nowhere.