Every goalie has aspects you can focus on and convince yourself they suck, if that's what you want to do.
Going by memory and gut-feeling without stats to back this up, but I think against Vegas 2018 it wasn't a case of Hellebuyck sucking, it was Fleury being lights out and the team being outcoached. The Jets just looked toothless as a team after the first games. Last year against STL on the whole you could argue Helle was the better goalie in that series, but I didn't like his performance. I just felt he let in too many soft goals in key moments and combined with the teams inability to cope with a loss of momentum, they lost those games. Again a lot of it falls on the team as well. You can't fall apart every time your goalie lets in a softie, which is exactly what happened with the Jets. In a way the STL series was weird, on the other hand the team really stepped it up and it was a hard fought series against eventual Cup champs, but at the same time you had this feeling that despite their ultimately pretty good performance, the team was fundamentally flawed.
This season Helle is having a great bounce back season and that's what really makes goalies good in the long term - how they cope with adversity and bounce back. Any goalie can be great for a 10 or 20-game stretch, but for the really good ones you know you can trust them to bounce back even when they are having difficulties for a game or for a stretch. I think that's the value what good starters bring to their teams, the team never has to try to gamble on the goalie or hope for a miracle stretch. They know what they have and they know they'll always bounce back.
I haven't been totally sold on Hellebuyck, but this season is convincing me he really is very good and the bad stretches are just momentary underperforming.