Not as bad as you think
This is my experience buying dozens of 'faulty' Hoover,
Candy, Indesit and Hotpoint washer dryers over the past 8 years. It used to be you could win faulty machines from eBay for 99p and most for less than a tenner, however, even machines with terminal faults such as drum bearings now go for much more so I very rarely bother these days.
First off I will agree that a washer dryer is a compromise over having separates, and if you have the space then I'd recommend dedicated machines, especially if you do a lot of washing. However, for single people or a couple, or those with absolutely room for only one appliance (or the 2nd space taken up by a dishwasher perhaps), a washer dryer is likely to be just fine.
First off, the design compromises: a washer dryer is a standard washing machine, but with dryer and condenser gubbins bolted on, and the associated extra (or different) programmes. This means the machine is just as good at washing as a wash-only machine, though sometimes some of the special programmes available on the standalone machine are missing and have been replaced by drying programmes instead (there are only so many selector positions available).
Secondly, as mentioned in the article,
the drum is simply too small to allow good airflow to dry a full load of clothes. Tumble dryers have much bigger drums because they don't need a watertight outer tub. This means that unless you only half-fill the drum with washing, you will need to remove some before drying.
Thirdly - and obviously - the machine can only be washing or drying at any one time, so if you need to wash more then you'll need to wait.
Finally, the dry cycle takes longer as it's also drying the machine innards
Now for the supposed unreliability: as has already been said, it's a standard washer with dryer gubbins, so the baseline reliability will be the same as the washer it's based on. In my experience the extra
water valve, fan and control module don't go wrong (they can, but I've never needed to replace them). The heating element
can break but this is no more likely, expensive or difficult than the main wash heater to replace. I've replaced maybe three at most.
By far the most common problem, and one that all washer dryers will succumb to eventually, is build up of lint which restricts the airflow and causes the thermal cut-out to trip on the heater box. In theory the lint is washed down into the outlet pump by the water flowing through the condenser tube/box, but a small amount always gets through and this accumulates over time with usage of the dryer function.
It's a smelly and unpleasant job clearing this wet and mouldy lint out of the condenser and fan but that's usually all that is needed (along with resetting the thermal cut-out) to restore full operation of the dryer again. No parts, just labour.
The older Candy/Hoover machines were terrible for clogging up, but at least it could be seen through the translucent plastic condenser tube on the back of the wash tub. The newer Candy/Hoover machines, plus all the Indesit/Hotpoint ones I've seen, have all needed the combined heater box/fan assembly to be removed from the top of the tub to see and remove the clogging, but this layout seems less prone - though not immune.
Finally three advantages of washer dryers: they can automatically dry straight after the wash cycle (keep in mind load size), and they're all condenser models so don't need to be vented. Better than most condenser tumble dryers, you don't even need to emtpy the collected water! Also they tend to be cheaper than separates.