I just watched Round 2 of his Highstakes Duel vs Esfandiari today and it was another masterclass.
His raw ability is definitely under-rated by mainstream fans and younger poker players.
Depending on the format, he is world-class. Obviously live big field NLHE tournaments being his preferred format, but he is very good in live headsup SNG format as well.
Everybody has favoured formats and everybody is better at some formats than others, but Hellmuth gets excessive criticism for not being good at cash games and online.
He doesn't play the cool formats of today (online HU & 6max Cash & live Super High Roller tournaments) and therefore his overall ability is brought into question, so yes, he is under-rated by a certain section of the poker community. For example, nobody questions Fedor Holz' overall ability, even though Hellmuth has been at the top for 30 years and counting.
Fun bits:
Check out King of the Hill from 2017 and an infamous hand vs Jungleman.
Check out NBC National Headsup Poker Championship, winner in 2005 and runner-up in 2013.