Sounds counter-intuitive, but bespoke WordPress themes can be a nightmare.
One of the great things about WordPress is it’s speed of evolution. And the fastest way to hinder that is to get a custom theme built for your site.
There are plenty of really good themes and frameworks available in the commercial space, and none cost more than a couple of hundred pounds. This stay supported for years, and evolve, so why pay thousands of pounds for a custom theme that’s out of date the day you deploy it?
WordPress is a framework built on openness and evolution, the idea that you may create a proprietary theme that blocks this evolution is a dangerous idea, and one that is pushed too often.
The rationale for using bespoke themes is also flawed. The theory goes that you can have a theme to make your site look exactly as you would want it. Except, standard (and premium) WordPress Themes allow that already. A good WordPress Agency can make any bootstrap-inspired theme look and feel like pretty much anything you might want. And for a fraction of the cost.
So the next time an agency suggests they are going to build your site using a custom (read: proprietary) theme ask them why and why they can’t use any of the standard WordPress theme frameworks. I suppose if you are an agency then the benefit is that the client might be tied into you? Maybe.
If you need custom elements inside a standard theme then use plugins and shortcodes to achieve the desired result. It’s cheaper, more flexible, easier to support and removes the agency lockin.