If it’s anything like the solar panel leasing scams in the US the downsides would be:

• the panels are low quality/old tech, not something you would end up with if you were buying them yourselves
• the contract covers your house, not yourself. So to sell you must get the new owners to accept the contract or pay for the panels outright first
• low quality installation that will likely degrade the roof integrity
• sneaky fine print where they can extract money out of you in various situations like maintenance
• absolutely no concern for viability of placement, installing panels where it doesn’t make sense. The focus is on installing the maximum number of panels so they get the most money

That’s definitely the problems with these contractors in the US. Might be different or actually legit elsewhere, but it’s a common “scam” for the last year or two and has boned a lot of people. IIRC it usually costs $20-30k to get rid of them or buy everything out to sell your house. Fine print!