Our club has been running since 2003 and we have a 40 shot FT course and a 40 shot HFT course permanently laid out in all weathers, plus a substantial number of "fun" targets, knock down and reset with no strings.
About half the targets are Knockovers, another quarter maybe are Gamo, a very few Umarex, and 20 or more home made to our own design.
What failures have we had? One of the Umarex ones had a weld go completely. The Gamo ones fall over too far, so when the reset cord is pulled, most of the tension goes into pulling the target towards you rather than lifting the faceplate on its hinge. It's a very easy problem to fix and we have added limiters to all of them so they can't fall past 45 degrees. Now the strings don't break and the chain links don't open out.
We've also taken out the mild steel clevis pins on the Gamos and Umarexes and replaced them with A2 stainless screws and locknuts.
Personally I like the older design of Knockover the best, the one with the bent S link, not the parallel bars like the Gamos have.
Our home made targets are very rugged. We have had one weld go, where a bush broke away from the back of the faceplate. When we made them we didn't plan well enough for maintenance so they have been altered as time has gone on to make them easier to strip and overhaul.
Of course if the target is fixed to a dodgy post then it may tip or the entire post give way, and we have had a few occurrences like that, but that is hardly the fault of the target.
We tend to keep our animaloid shape targets with full size kills and use our plain round knockovers for close range mini kills. This has led to the minis getting a concentration of hits in the centre of the paddle and some of them are now distorted and will soon need new paddles, or a dressing with a big hammer. Not bad after ten or more years.
Our normal preventive maintenance is a thorough brushing with chassis grease on the moving parts, twice a year.