A couple of disadvantages to it are:

• It's a very new method. Older browsers will not be able to understand your code if you use it, unless you include a polyfill - so make sure you include one..
• On a completely different track, since it's such a new method, I wouldn't be surprised if a substantial number of developers haven't heard about it - using .at may confuse some of them (and cause them to have to look it up). Not that this means that you shouldn't use it, but it's something to keep in mind.

You don't need a for loop at all with JS' Array type because it's an indexed type (which means it has O(1) lookup for random/arbitrary-location reads and writes) when you use its [] indexer. The .at method is just an alternative way of performing random reads (and allows you to pass it around as a function reference, which you can't do with the indexer, at least not without creating a new function with a closure).

You only need to iterate over a collection when that collection's type only supports the Iterable interface (such as when using objects representing a Generator function).

Anyway, you can reimplement at in a single expression: Array.prototype.myAt = function( idx ) { return idx >= 0 ? this[idx] : this[ this.length + idx ]; }

(I just learned you can't usefully use => to add functions to prototype as it won't capture/bind the the correct this, so using => is only okay with static-like functions that don't use this, but otherwise you need to use the long-form function() { ... } syntax).

...or more thoroughly:

/** Reimplementation of https://tc39.es/ecma262/multipage/indexed-collections.html#sec-array.prototype.at */
Array.prototype.myAt = function( index ) {

   const asInteger   = Math.floor( new Number( index ) ); // https://tc39.es/ecma262/multipage/abstract-operations.html#sec-tointegerorinfinity
   const actualIndex = asInteger >= 0 ? asInteger : ( this.length + asInteger );
   return this[ actualIndex ];
};

//

const arr = ["JavaScript", "HTML", "ES2022", "CSS", "Chrome", "W3"];
let res = arr.myAt(1);
log(res); // output: "HTML"