What are the risks of staking?

In SKALE Mainnet Phase 2 (started October 1, 2020) there is no actual risk to the stake, as the main penalty for validators at this phase is bounty reduction if a validator doesn’t perform well. SKALE asks for at least 96% uptime. The validators that are selected for SKALE are up 99 or 100% of the time, so the risk of bounty reduction is low.
In Phase 3 or later, the stake will be slashed if a validator exhibits malicious behavior. During the permissioned node phases, SKALE is working with exceptionally high-quality validators, so this is very unlikely to happen.
In Phase 4, when the network is fully open and permissionless, slashing will come in full effect and the presence of slashing will motivate Validators to avoid malicious behavior and will also drive delegators to do proper due diligence and work with high-quality nodes.