This has been my experience too. I was looking forward to the feature and upgraded straight away to try it out (something I'd never normally do!). But I've found that response to commands is flaky, there's a noticeable performance impact, and, worst of all, dictation doesn't seem to be supported in non-Apple applications, e.g. Slack. Pretty disappointing.

I'm still using the "basic" dictation feature, which works well enough, but doesn't support editing.