Looking at a ZyXEL VMG8924-B10A for £48 which is used but comparing it to a TP-Link WD9970 which also has a Broadcom chipset which is £35 new.
Is the ZyXel worth the extra cost?
There's also a Billion 8800NL which also has a Broadcom chipset and is £40 used.
Unfortunately it's not that simple. The TP-Link and Billion models you mention have both been out for years and have both changed chipsets over the years (for the worst).
The TP-Link WD9970 used to have a Broadcom chipset.
It has many revisions and I believe the latest (v4) has a Mediatek chipset, not Broadcom.
The likes on Amazon are very likely to have the latest revision.
TP-Link also strip back the accessible stats on most of their modems.
The Billion 8800NL also has 2 revisions.
The Billion 8800NL R1 had an excellent Broadcom BCM 63168 chipset. It hasn't been supplied for years now.
The currently supplied version is the Billion 8800NL R2 which now has a cheaper and not so good Broadcom BCM 63381 chipset.
The R1 and R2 look different.
This is called bait & switch.
You release a model with premium components, get great reviews online, then using the same model number release later revisions with cheaper parts.
People buy the product based on the great reviews from when the product was released.
People tend not to review products years after release or to review later revisions of products.
Zyxel have done similar by going from the VMG1312-B10A to the VMG1312-B10D.
The B10D has an interior chipset.
In Zyxels defense it's a slightly different model number so you can actually see for definite what you are buying.
The 2 also look very different.
The Zyxel VMG8324-B10A or the VMG8924-B10A are great.
They are essentially the same device but the 8924 also has 5Ghz AC WiFi, while the 8324 is 2.4Ghz only.
My favourite was the Zyxel VMG1312-B10A.
It was the most stable and highest syncing modem I ever used and I also loved the shape and colour and how it could be wall mounted next to my master socket.
It's basically* a black version of the OpenReach modems in styling.