You should definitely avoid Dialogflow CX if you are building a large one-and-done FAQ chatbot. A good example of such a bot is the one which selects a single article from my website based on the Dialogflow topic you are interested in.
By large, I mean a bot with 10s if not 100s of intents.
One and done bots answer a single question (and provide a link to another web page, for example), but are not built to handle follow up questions from the user.
Often, these two are the same thing. It is hard to manage if large bots answer follow up questions, and one-and-done bots can be easily scaled to answer hundreds of questions.
Another rule of thumb:
If your bot will benefit from the FAQ generator in my BotFlo app then that is probably not a good choice for CX. BotFlo converts a CSV file with all your intent information into a Dialogflow agent ZIP file in a single click (i.e. you use the CSV file like a content management system for your bot). Since Dialogflow ES is perfectly capable of handling such a bot, you wouldn’t need the additional complexity and cost associated with CX.