Once upon a time, bespoke CMS solutions and PHP frameworks were the best things for a client. This time, however, has now passed. The use of a bespoke CMS is an archaic practice which can more often than not, be detrimental to the clients business interests. Here's why.
You will be dependent on the author of your bespoke CMS
If you purchase a bespoke CMS, you are reliant on the people who coded it. It may well be written in PHP, but it's not as simple as just finding another PHP developer. Without extensive documentation, the code might as well be jibberish.
Sure another PHP developer can work with with the code base, but they will never know it as well as the original author and they will take a hell of a long time to comprehend it properly (guess who is going to pick up the bill for that). Whereas with an off the shelf CMS or framework; if you move to another developer who specialises in said system, they have all the prerequisite knowledge required to work on it right away.
There are excellent training resources (Codeschool, Laracasts, Treehouse) for open source CMS & frameworks; these do not exist for bespoke frameworks.
Do not underestimate how much developers hate inheriting badly documented bespoke code. If you're hiring a freelancer, you will be paying a premium for what they have to endure. Or if you have an internal team you are going to force to work with it, you are going to have morale issues very quickly.
I have seen developers turn down jobs at digital agencies, based upon fear of what they might inherit if they were joining up - bespoke CMS solutions are an excellent example of the kind of thing that induce this fear.
It is not Tried & Tested
The bespoke CMS may have 100 happy users, but the mainstream CMS has many, many thousands (or even millions) of happy users. Many people are sharing their experience with the system in online communities etc., something custom CMS won’t have. Because so many people use the CMS, bugs will be found and fixed much quicker than on a system that has far fewer users. This high level of usage means any holes in security, glitches, performance issues and non-user-friendly quirks are very quickly ironed out.
You won't get the support you need
If you need help on a bespoke CMS, who will provide it? I have worked in many agencies; I have yet to discover one that makes real money off support contracts. The result? Support slips to the bottom of the pile underneath new projects that generate more cash.
So, your probably going to have to look for support elsewhere or internally. Which, brings us back, to dependence. If you have used a mainstream CMS or framework finding this support is going to be much easier.
How secure a bespoke CMS can be is a separate matter of debate. However. When a security exploit is discovered (and, it is a case of when; not if), how quickly could the devs who made your CMS push out a security patch? Would they be able to push it out automatically or would they be able to do it as and automated update? Either way, It's highly unlikely they could patch up the exploit as quickly as an open source team.
The bespoke solution will rarely be as good as the mainstream alternative
Laravel, at the time of writing; the most popular PHP framework on the net, has 338 contributors. That's the combined effort of 338 developers. A small internal dev team, building a bespoke framework or CMS, DO NOT know better than this many people.
On the modern web, the use of bespoke CMS and frameworks is irresponsible at best and benefits nobody, apart from the team who developed and sold it.