I run older out-of-warranty Greenmechs and know them pretty well (with a massive amount of my knowledge gained hands-on with direction over the phone from their very helpful technical support). Two of my machines cost less than one new equivalent (whatever the manufacturer), but they do require good maintenance (but so does a new one). I certainly wouldn't consider them worthless/unreliable after 3 years (with the exception of cheap Chinese crap, I don't think there are any manufacturers who supply woodchippers into our marketplace that are as poor as you suggest).
When looked after properly chippers are not unreliable - we've got a 1928 Safetrac with over 3000 hours on it - it isn't pretty but it turns wood into chip. My thought process for operating this way is that if/when I suffer a breakdown, I'm not totally reliant on that single machine. It gives a bit of breathing space in the event of a serious failure. Big expensive failures are pretty infrequent in reality if you are handy with the grease gun and keep an ear out for strange noises.
It becomes harder if you are sending lads out operating older machines - the maintenance regime needs to be top notch then, with potential issues picked up in the workshop before they aren't noticed out on site... And a brand new machine can become unusable just as quickly as a second hand one if abused/not maintained.
As for the subject of this thread, I take my hat off to Forst. They stepped straight into the marketplace with a viable product and appear to stand behind it/support it wholeheartedly. If I'm honest I think their appearance forced other manufacturers to up their respective games, bringing new machines out with further advances in design that we perhaps wouldn't have seen so quickly.
I have read/heard about various issues with Forst machines, but have seen/experienced issues with all of their competitors at one point or another. It has been said before many times but is worth reinforcing - parts availability and dealer/manufacturer backup is worth far more than the badge/colour of the machine you end up operating. That is what will get you back up and running WHEN you experience issues.