To be pedantic - I'm thinking mould/moss build up on the panels, as you would expect on any roof, and degradation in the wiring between panels, as you would expect with any outside installation.
So basically, you have to get a roofer in to clean the panels, and the cabling might degrade, before the outlay is recouped. Or a panel could fail randomly.
Solar panels have a smooth finish and, as such, make a poor surface for moss to grow on - when have you aver seen moss growing on the glass surface of a window?
The cabling will happily survive the 25 year term of the contract, likely longer.
In the end, deals like this will drive the solar industry forwards in the UK, just like similar deals have in other countries i.e Germany. Sure, they aren't for everyone, folks who are able to afford/want to finance an installation of their own are well advised to do so, however, a large portion of the population can't or don't want to make large financial commitments like this - 'Free Solar' packages are aimed at those people.
Today solar installations are unusual in the UK and because of this people notice they are there. Over the medium term, solar installations will become normal and they'll be no more 'noticable' than windows or front doors or TV antennas.
In my view, anyone considering a 'free' solar installation should make sure that there is a buy-out option, so they have to freedom to purchase the installation at a later time if they want to.