But NE has safari view controller which supports ad blockers and also Mercury and other parsers. So beside FOSS what’s the downside ? It supports Tv and Watch

If you must have compatibility with tvOS and Apple watchOS as well as macOS and iOS/iPadOS, NE certainly will do the job. (Can’t say that I’ve seen it work with ad blockers, but I’ll take your word for it.) As for other parsers, I’m guessing most NE users stick with its default settings, but have no data on which to base that guess.