I got 99 problems but my WiFi ain't one

The trouble with making your own aerial is they can have too much gain which can end up with a signal that is stronger than is legal. The classic example used to be making directional aerials from Pringle tins, too much power in a single direction

True in theory but its almost impossible to do in practice. Particularly on 5ghz where the EIRP cap is much higher.

On 5ghz you can have an EIRP up to 53 and on 2.4ghz up to 36. Well, you can go higher on 2.4 but you have to reduce transmit power a bit depending on how high you go.

This means that on an average wifi card, let's go with an intel 7260, you'd have to have a pretty crazy antenna to go over the top. Roughly 22db gain for 2.4ghz and a theoretically impossible 41 db gain for 5ghz.

In my case, the antennas I make that outperform stock ones are maximum 3db of gain as they are omnidirectional and monopole without concentrators or reflectors.

Edit: Worth mentioning that most aftermarket antennas are advertised with made up gain figures. A significant number of the bestsellers actually attenuate.