The risk is real and I have personally seen a Linksys ADSL router with WiFi get compromised over the internet with no available firmware updates. I would say the Billion most likely suffers from this flaw
Billion devices are on the list and the flaw dates back to 2013 so who knows. There will almost certainly be a bunch of other flaws especially in it's WiFi implementation.
I have also seen a TalkTalk supplied router get compromised most likely again over the internet. Finally I have also seen a Billion WiFi access point get hacked. That said most people would likely not have been aware of either compromise as they where quite subtle in their effects. In all three cases the hardware was perfectly functional, just no updates where available. I have no more data because the hacked TalkTalk router was the last straw and everything was replaced across the family by that time with stuff that is still supported even four years later. Applied an update last weekend, and I am expecting an update for the Edgerouter any day now for a potential denial of service attack.
A Vigor 120 v2 ADSL modem, an Edgerouter X-SFP and a UniFi AC-lite is less than a years internet subscription and all under maintenance and likely under maintenance for many years to come. If you upgrade your internet connection you can just swap out the Vigor 120 for something that supports your new internet connection say a Vigor 130 for VDSL etc. You would need a rather expensive internet connection (better than a 330/50 FTTP to cause problems for the Edgerouter).
Cheaper would be a Vigor 120 v2 ADSL of eBay for £15 and a Mikrotik RouterBoard hAP AC Lite, probably £75 all in and again likely to be supported for many years to come. Heck if you are happy with the WiFi from a BIllion 8800NL then a MikroTik RouterBoard hAP Lite with a Vigor 120 would do the trick for under £40.
The point is devices with long term support are cheap, so why take the risk? That said most ISP will send you a new device out every couple of years for the postage which is better than something that is ancient though not exactly environmentally friendly. I also feel that simply applying updates is a lot less hassle than configuring replacement kit.