People say Shanks cheated and didn't fight fair against Loki. Though Gaban clearly said "Shanks" did that, not "Red hair pirates" as whole, then Gaban either didn't explain it well, or Shanks alone caught Loki offguard which would be still a great feat considering Loki's durability.
That's the argument that people use against Shanks.
However, some people who says this, also say that Shanks is a swordsman and he doesn't fight dirty. That we see all of Shanks's abilities, he should fight without cheating, only by using sword.
I believe Shanks can fight dirty, which aligns with what Shanks said before;
Shanks: "We're pirates! We don't play by the rules!"
Zolo: "I get it, you're just a blood thirsty brute!"
Alber: "I've never understood why anyone would stick to traditional forms and techniques in a real battle."
Zolo: "I guess you have a point, you never claimed to be a swordsman after all."
Which means Shanks isn't just a swordsman, he won't be limited with being a swordsman because he can fight dirty as some people say about the Loki fight, thus he isn't limited with the WGS title.
We also know Shanks has CoO killer ability that has nothing to do with swordsmanship, which is another technique that proves Shanks can't be limited by swordsmanship and WGS.