He's amazing during the regular season. It's just during high pressure playoff games that he disappears.
It's a tough call when all his underlying numbers are saying he was a dominant force out there the last two playoffs and ran into hot goaltending. Maybe its just a difference of opinion that when I was swearing at my TV at Galchenyuk and Dermott and Sandin blowing games while it seemed like depth guys like Mikheyev was intentionally trying to get the best chance possible without scoring (he has the record for fewest points (0) in the NHL in the most minutes the last two years) and marveling at Price's performance, you were focusing in on Marner for some reason and I'm not sure we saw the same thing. Eye tests will always be this way as they are unreliable so all we have are the stats.
If I try to get the biggest sample possible and rank Marner against the 106 NHL forwards who played at least 200 5v5 playoff minutes the last two playoffs, here is how he ranks:
Shot differential: 125-88 (58.08%, 13th best out of 106)
Scoring chance differential: 123-90 (57.16%, 10th best out of 106)
High Danger scoring chance differential: 45-19 (70.45%, 1st by far out of 106)
Expected Goals%: 66.29 (1st by far out of 106)
A combination of these numbers are almost always fairly good for assessing how good a player was out there, the bigger the sample, the more accuarate they will be.
To give you an example of how these numbers usually work, lets sort it out by right wingers.
5v5, the top right wingers ranking the last two playoffs (200 minutes+, 28 RW total):
Shot differential:
1. Tuch
2. Kucherov
3. Voracek
4. Svechnikov
5. Marner
Scoring chance differential:
1. Rantanen
2. Tuch
3. Pastrnak
4. Marner
5. Kucherov
High Danger scoring chance differential:
1. Marner
2. Tuch
3. Kucherov
4. Voracek
5. Gallagher
Expected Goals%:
1. Marner
2. Tuch
3. Kucherov
4. Voracek
5. Smith
These lists generally hold the best performing players (hense Kucherov being on every list for example)
As far as Marner goes, I just don't think he's been given a fair shot here. No one denys the goaltending he was playing against was great either with his 8 points in 12 games.