While Handoff uses Bluetooth, making calls and SMS forwarding do not. Making calls only requires you to be on the same WiFi network as your iPhone, Bluetooth is not involved. SMS forwarding I believe goes through Apple's servers (the SMS comes into your phone and then your phone forwards it through Apple's servers back to your devices) -- you don't even need to be on the same WiFi network.
So SmileyBlast! is right, this increases your security risks. If SMS forwarding was compromised, things like two-factor authentication that use SMS would become vulnerable. And attackers who penetrate your WiFi network could potentially make/receive your calls.
It's not as simple as having your iCloud user/password, though. I know at least the SMS forwarding requires you to enter a code on your iPhone that you receive on your Mac when you enable SMS forwarding. I couldn't even use it with my old Mac Pro 1,1 (hacked to run Yosemite) without contacting Apple with my serial number to enable my machine. So there are some safeguards in place. But a determined attacker could get by these things. And of course if an attacker compromises your Mac or other enabled device, they can now make phone calls from it and send/receive SMS.
Personally, I'm aware of these risks, and still choose to use SMS forwarding and calling from my Mac. There's probably things Apple can do to tighten things up. For example, whitelist/blacklist specific SMS numbers for forwarding from your iPhone. And I only use the calling part to notify me when I'm receiving a call or missed a call, and just use my iPhone for actual calls -- would be nice if you can restrict it to being only a notification mechanism.