Platforms are a reasonable choice, and for plenty of sites they are the right one. What you accept in exchange is their pricing, their update schedule, their plugin ecosystem and their limits, plus an editor with a hundred options where your team needs six.
A custom system has fewer options on purpose, because the ones it has are yours. The cost is that somebody has to build it, which is why it makes sense as part of a build rather than as a thing bought on its own.