i never heard of them failling! there just solid bits on metal which wear in time and get noisy!
depending on mileage they do get noisey..... but if left no harm will be done!
i have 205000 miles on my 8v mk 2 gti and ive never replaced the tappets....... they are abit noisey but nowhere near as noisey as the fords!
they can be replaced with head still on just take the rocker cover off and take the cam out and remove them all!
if you need to do valve oil seals then either the heads got to come off or you need an air compressor which connects the the spark plug hoes!! and a valve compresser to do them with the head still on...... to do the valve guides the head has to come off!
if the engines not smoking not burning oil and runs ok and the slight tappet noise bothers you just replace the tappets.... if it starting to smoke and or your burning alot of oil over 1 litre per 1000km then get a compresion test done to see if bottom end is sound......
if it is just replace the valve seals at the same time as your replacing the tappets..........
