Lido is a secure liquid staking solution for a number of reasons:
Open-sourcing & continuous review of all code.
Committee of elected, best-in-class validators to minimise staking risk.
Use of non-custodial staking service to eliminate counter-party risk.
Use of DAO for governance decisions & to manage risk factors.
Usually when staking ETH you choose only one validator. In the case of Lido you stake across many validators, minimising your staking risk.
Lido has been audited by Quantstamp, Sigma Prime and MixBytes. Lido audits can be found in more detail here.