Sometimes the BuiltWith extension just says "The website says we are forbidden to do a lookup on it."... What does that even mean? This is only supposed to show a concise summary of what's in _View Source_. There's no way to "forbid" that.
Even when it shows information, most of its UI is devoted to upsell attempts. It's not even clear what they offer for the $99 price tag, but that's likely more than I'd ever pay for a simple browser extension.