How safe is it to delegate to a stake pool?

Delegating your stake to a stake pool is 100% secure.

1) You should never transfer ADA to a stake pool. If a stake pool asks you to send them funds in a transaction, please DO NOT PROCEED and report that to us. 

2) The delegation mechanism allows you to have full control over your funds. You delegate ONLY your right to participate in the protocol producing blocks. Thus, when you delegate to a stake pool, you only allow them to produce blocks on your behalf, but you NEVER give up control of your funds. 

3) When you delegate, your funds are NEVER locked, you retain full spending power over your funds at any time. 

4) To delegate your stake to a stake pool you need to do it from the Delegation Center in Daedalus app. 

5) If the stake pool that you were delegating to, disappears, gets retired, stops working, or has any other bad performance issue, you can just re-delegate your stake to another stake pool.  Your funds are always secure under your control. 

6) Stake pools do not have any control over the rewards distribution amongst its delegators. Any rewards earned by delegating your stake are distributed automatically by the protocol. Stake pool operators have no control over this process. 

7) Delegating is secure and does not put your funds at risk. 

8) Rewards from a given Epoch are distributed 2 Epochs later and those are added to your wallet balance and it is automatically delegated to the stake pool that your wallet is delegating to. 

See also: The delegation cycle

9) Once rewards are added, you can always spend those rewards too as part of your wallet's balance.