Yiazmat is extremely difficult to defeat in-game, so since you went through all the perils of completing this sidequest, now your opponents have to go through the perils of battling this nearly indestructible ultimate boss
That said, I'm not too surprised because the card is pretty powerful
Recent sets have introduced build-arounds like Insidious Roots, Kishla Skimmer, and Quintorius, History Chaser, inching this strategy ever closer to critical mass
Once that happens, the value of X won't change later in the turn even if the commander creature's power changes
Perhaps Erase is a good card against this particular mono-red deck, but that would only follow from logic along the lines of: "This matchup is very favorable for me in the absence of Outpost Siege, so it's worthwhile to sideboard a narrow answer card because it prevents one of main ways I can lose." Such specific and thoughtful analysis of the matchup is great, and it might lead you to play with one or more copies of Erase in your sideboard
I've never met him, but some reputations precede an introduction