[jacquesm]

Wago use a spring loaded contact to touch the wire and there are at least two of these in series for every connection. This works quite well for currents up to about 20 A, and if the Wago is applied properly: straight wire, right thickness and proper length stripped. Obviously if you mess any of those up all bets are off. But assuming proper installation that 20A is still a potential issue: lots of gear has a multiple of that for a short time when starting up. That's when contact resistance starts to become important, and as connections age mechanical resilience comes into play and what was borderline before may become a problem. Contact resistance is ideally constant but as connections age they are pretty much a factor of how much copper is in contact with each other and this is where wire nuts do a better job. So a brand new Wago splice and a wire nut splice will probably have approximately the same contact resistance, but several years down the line it may well be quite different.

Motors and other large inductive loads are especially nasty in this respect, which is why there are several 'curves' in use for ground fault interruptors. The most common, the B curve will do a max or 3 to 5x for a short period of time before it kicks in, but a C or D curve can go much, much higher. And then that contact resistance (which wasn't such a huge problem so far) suddenly is a problem. Now you're generating serious Wattage in a small space that has no good way to get rid of the heat. Imagine an elevator motor or a shop lift or or something like that. This sort of application is where I would never use a Wago. But for regular low power stuff in my house I use them all the time, lights, bedroom outlets and so on. But my 17 KW Solar inverter is using crimped on joints and I checked the torque on the screws in the plug to make sure it's all up to spec. Those are not things to mess around with and hope it will hold.

If you want to have some fun take a FLIR across an older exposed installation, it will definitely help to visualize what contact resistance will do.