I've tried Video DownloadHelper as a YouTube-DL alternative to download a paid video I had rented on Vimeo. The extension offers to downloaded the 1920 x 1080 pixels DASH stream, and downloads it fast, but then it launches a transcoding (called an aggregation), even though I've disabled "Force H264 tuning in aggregation", which would have taken hours. It turns out the free version adds a watermark, which requires post-processing and transcoding. My understanding is that you have to pay €18.50 for a premium license to get fast downloads without transcoding.

I have turned to Downie 4 by Permute's developer Charlie Monroe, which is also paid at €18.99, but has the advantage of being up front about it. Alternatively it is included in the Setapp subscription service. It has a cleaner interface and it works with Chrome and Firefox as well as Safari and Opera, alas only on macOS.

tl;dr: Video DownloadHelper should clarify its freemium business model on its official website and in its browser extensions.