Dubas like most GMs had good moves and bad ones. You are ultimately defined by success and the Leafs had lots of regular season success without a lot of playoff success. I do think that the division they play in has been part of the problem though for years and it's been a bit unfair to judge the Leafs playing in a division with Tampa/Boston/Florida over the last few years. The Atlantic division (*caveat for the COVID divisions) has won 3 of the last 6 Cups and 7 teams were in the Cup finals that play in the Atlantic over that time.
Tampa x 3, Florida x2, Boston x1, Montreal x1
I think that really don't get factored in enough for how much of a mess the division has been. The current playoff format has forced the team to be in a tough gauntlet right out of the gate despite being a consistent regular season team.
But it doesn't matter. They only won one round and Dubas did a poor job adapting to the necessary style of play to win in the postseason.
I don't think he's a bad GM, but he's absolutely more in the middling range. I do think we have to take into account that we don't know how much autonomy he had to make moves under Shanahan. Similarly he went to Pittsburgh and was told to try and make it work with their old core. Blaming Dubas for Pittsburgh being bad is dumb because the team should have been a rebuild. He chose to take on that job though so again, its on him for putting himself in that role.
So far I've actually been quite happy with Treiliving's job in Toronto. There are gripes with how the roster is constructed....I still don't understand why Reaves got the term length that he did...we havne't been able to find good depth scoring and the blueline is horribly missing a guy who can create offense, but I think he's done a good job working with the framework he has. His free agent signings have been fine but we also haven't seen him take on an important contract negotiation yet to see how he handles Marner/Tavares/Knies. He held tough with Robertson and I wonder if that was a contract that Dubas would have given more money to to try and sign earlier.