Most directly and most likely, it can hurt your credit score if you miss payments, since it's being reported regardless of whether or not it benefits you. This is a risk for you but a benefit for landlords, since it gives renters more incentive to actually make on-time payments.
Secondarily, there's a small risk that Credit Ladder or one of their partners makes a mistake and you have to spend some time correcting it. They do seem to have reputable partners, so it's likely that mistakes can be corrected, at the cost of wading through customer support.
Finally, it's important to look at where they get their money.
Credit Ladder charges directly for premium memberships - it appears that those are basically a sort of group discount thing. None of the benefits seem super awesome, but the cost is low enough that I think it could be worth it for some people. So that's one way to make money. Risk here = this service doesn't make enough money and they might phase out the free tier.
Now, source of money #2 - your information. According to this source, "within the Privacy Policy, the company states that [they] may use the personal information collected about you to “provide third parties with statistical information about our users – but this information will not be used to identify any individual user.” It’s not clear if this is the intended monetization of the company, it is a consideration to the user that this information may be sold." Risk here = they might get hacked or they might sell your 'non-identifying' information. There's no details as to whether or not they are actually doing this, but they have the right to do so. If they got even 10% of the renters in UK using their service, then this data would be quite valuable.
