Yes Karwa buses are safe and also economical.
They separate female and male passengers.
The first 6 front seats are for the ladies. So if there are men sitting there, they are required to give way.
If the driver is (), they tend to volume down the aircon. I don't know maybe to save gas, but why? The company pays for the fuel. So expect a "warm ride" with them. But mostly there are more non-() drivers now.
SmartCards are available in some supermarkets and all bus stations @QR20, refillable. Without it, you need to pay QR10 per trip. You save QR8 per trip with a SmartCard (QR2 per trip only). Bring a photocopy of your Qatar ID when purchasing one.
HTH