Screw terminations can creep when subject to heating and cooling as part of normal operating conditions. Lever connectors work on a constant tension principle and should be reliable in the long term. That they can be considered maintenance free when suitably enclosed would suggest a considerable potential lifespan.
There are lever connectors which have been subjected to rigorous testing in order to gain approval from numerous regulatory bodies around the world, although I stick rigidly to one particular brand There are also unbranded lever connectors, of unknown provenance, sold through online marketplaces, which I would recommend avoiding at all costs.
I was sceptical of the spring loaded stuff when it first came out, but that was many, many, years ago and it seems all of the field evidence is reputable spring-loaded terminals are more reliable for the average user than screw terminals, and many are classed as maintenance-free when suitably used/boxed.
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