Is it completely safe to have multiple transactions sent to the same address? I have read that "using" the same address multiple times could be unwise. Does this refer to "sending from" or "receiving" or both?
This is a weird question because you have to send from the address you receive from. If you receive all your bitcoin to the same address, you will inevitably be spending from that same address as well.
When you receive to the same address, it ties together the addresses that are paying you. If you are running a business, this could allow chain analysis to figure out who your customers are. This might be bad for your business, and it might be bad for your customers. Giving customers a fresh address for each transaction will give you and your customers the most privacy.
When you send from the same address, chain analysis can determine how you spend your money. Also, consider addresses used for "change" outputs. If you re-use the spending address as a change address, it makes it very easy to detect which output is the amount being spent, and which one is your change in the transaction. This now makes your purchasing habits even more obvious to anyone who is watching.
For best practice, always use a new address for receiving bitcoin, and always use a new change address each time you spend it.
You can receive as much as you want to an address, but you should spend from that address only once.
The reason you want to spend from an address only once is that this exposes your ECDSA public key as part of the spending sigscript. Should a vulnerability for the secp256k1 curve be found, your funds would be in danger. This is somewhat unlikely, but it is possible.
If you haven't spent any amount from an address, you would be protected by sha256 and ripemd160.