1. It treats all revenue as equal
A sale of your worst-margin product counts the same as your best. Optimise hard toward ROAS and the bidding will find you the cheap, thin-margin volume.
2. It is reported before returns
In fashion especially, the interface number and your bank balance describe different businesses.
3. Raising it always works, briefly
Tighten the target and ROAS improves while volume collapses. Technically an improvement; commercially a retreat. This is the single most common way accounts get "optimised" into decline.
Better
Break-even ROAS as the floor, maximum allowable CPA as the working target, and profit — not ratio — as the thing you are trying to grow. Calculate the floor.