The ADF system is quite good for the purpose for which it was developed around the time of WW2: providing crude but usable guidance for long distance flying. The NDB transmitter is a simple thing to make and to keep going, and the more kilowatts you pump into the bottom of it the longer its range will be. It works well over flat open country, the sea, and over long ranges (hundreds or even - with huge amounts of power - thousands of miles).
I routinely tune in an NDB when flying long VFR legs across e.g. France, for both backing up the primary nav (GPS) and to give me a well known clear waypoint for the flight plan. I do this a lot less now because I do long flights under IFR and IFR (airways) is all GPS - GPS (RNAV) is in fact mandatory.
Unfortunately NDBs have gone out of fashion for enroute nav a long time ago, due to VORs and more recently GPS, and tend to be used mostly for instrument approaches, and that is where they fall down badly. When you are close to the beacon, say tens of miles or less, you are entering the part of its radiation pattern which is affected by terrain/coast assymetry, electrical storms, god knows what else, and it's easy to find one's track 30 degrees off. The approach designers know this of course so NDB approaches are designed with loads of latitude for lateral errors, often using DME to ensure that if the radiation is disrupted so badly you end up way off track (but have followed the DME stepdowns) you will probably not hit any hills (but may not be able to land, which is "OK"), but the end result is an instrument approach with a decision height like 800ft which is basically useless for the average British Warm Front kind of weather...
Airliners use inertial navigation now, the new ones with GPS, and they totally ignore NDBs enroute. They also tend to ignore NDBs on approaches, flying the approach using the INS (FMS) and only checking the NDB at the start. However I think there are enough dual-NDB approaches in Russia to keep the ageing CAA ATPs happy
To top it, of all the avionics you can stuff into a GA plane, the ADF is probably the least reliable. The current KR87 is generally OK but most of the others were crap, and expensive to repair. It is common to find, on the average spamcan, that the ADF is duff and nobody wants to pay the £3000+ to get it fixed.
AFAIK the current proposal is to do away with the requirement for the ADF for IFR in CAS i.e.
enroute. Most European countries never had such a requirement in the first place - it's a peculiarly British=Superior thing - and just looks plain silly.
I don't see the ADF going away on instrument approaches in Europe, where it is widely used as a locator.
I would keep my ADF working but won't be sorry to see enroute NDBs disappear.