Range extenders can sometimes make things worse (EG in dense populated areas) because it takes a congested network hurting performance and congests it some more.
The dual radio extenders don't halve the bandwidth like the really cheap ones do.
That is not the problem I was talking about (but indeed still a problem). During my student job, I often ran in to cases where a person, let's call em' Alice, lived in an apartment building. Alice didn't have good wifi reception in say her bedroom. They talk about it with their neighbour, Bob, who says they had issues too and so bought a range extender. Alice decided to do that. They come home, install the access point, but now still have issues. I get called up (as IT tech), and I run a network scan, and find more than 70 networks in their building. The problem: Everyone has wifi blasting out full, and then to get even the smallest bit of reception, they need a billion range extenders on to of that, and now their spectrum is congested with re-transmits of re-transmits and announce packets. And because their extender re transmits everything it gets, It will try to re transmit all those packages too on another band, just making the problem worse. If everyone in the building starts doing that, it is a miracle you can get a list of networks in the first place. This is why those distributed-access-points spaced around the house using ethernet or power-line communications to talk to a core box is gonna give you much better performance. In fact, the way to alleviate such congestion problems, I think (not an expert), is to place much more access points (not extenders), but lower down their output power and put them in different bands. Kinda the same thing they do with nano/attocells in 4G/LTE/5G