Very. All online transactions use proper protocol and encryption and as cards are usually used in their entirety for one deposit it's never likely your number is co-opted and used again.
Standard online security precautions should also be taken, of course, like never letting anyone see your my paysafecard username or password and keeping your e-voucher PIN # safe before using.
Again, as even the pre-paid paysafecard Mastercard requires no banking or credit information your personal information is always protected.