Lmao.... What's the disadvantages of that versus the typical multi ring crank in the front?

OK, the 3 internal ratios, relative to the external chainring ratio are fixed. % of the middle gear..

3 speed hubs tend to be fixed on, with a nut, [like a fixie, so tools are common and, Hip.]
rather than a hollow axle with a QR running through it.

But , you can grab the low range even when not turning the crank at all .

they are nice features on Bike Friday's NWT a touring bike that packs in a suitcase..