Many electronic components have 'steel' connecting leads..
In our climate, these corrode very quickly, and fail..
These devices have the minimum thickness of copper-track on their boards. Slight dampness, combined with them being 'always on' (as far as small currents are always present) means the copper tracks migrate, leaving bare substrate.
Better weather proofing, and a spray of circuit board lacquer, can extend the life of the unit..
For the battery itself: These are not of particularly good quality, to start with. If you get more than 18 months (around 500 charge-cycles) you are doing well.
I had a railway hut with a solar light on the window ledge (inside) at Tamworth, which was 4+ years old, and still worked fine. - An old solar light, which had failed. Re-wired, and a new (old) cell.
PhilP