Question about Netgear Nighthawk X6S mesh extender

Basically, all currently available repeaters are "reliable". Unfortunately, we can't really say whether this product is more or less reliable than your current installation.

The Devolo Gigagate is not 'just' a repeater, it is a real dedicated bridge between a transmitter and a receiver.
I use one with total satisfaction, the speed is very high and without interruption. It's better than PLC (wired) but the distance between the devices has to be "decent": 10-15 metres indoors with dividing walls, 30 metres and more in open field (few or no walls).
I've also had wired Devolo Gigagates, but the reliability is less good.
A repeater is NOT a bridge, in the sense that the flow is shared between all the devices connected to the wifi, whereas with a bridge, there is a "virtual wire" connection between the transmitter and the receiver.
Some routers / repeaters can also be configured in bridge mode, but they are either more expensive or less efficient than what the Devolo Gigagate starter kit (the 2 black boxes) offers.