Are you still undecided over whether to go for that custom CMS build? It wouldn’t be the first time that I have asked “Why not WordPress?”, and the figures below just go to demonstrate how much of a popular platform it is.
Yes it does have its short comings, security having been one of them in the past. With the proper management and maintenance most of these can be overcome. I agree there will always be exceptions, there may even be a preference for another type of platform. But when it comes down to WordPress or a custom build I find the custom solution very hard to justify.