I'm sure they thought it through properly before releasing an app for it. There are huge risks involved for any bank if they do not take measures to secure their channels. Then there are also the measures you need to take. Telling whether it is a trusted app on android might be a concern, but if you follow the links from the FNB website, you should be fine.
Even if you don't have the app installed, they STILL have access to your OTP device if your phone gets stole , and although they might have your login name, a password that can be cracked in 3 attempts is definitely not secure (back to the security measures you need to take). So although I'm not saying it's completely safe, I personally feel it's safer than cellphone banking, which is also accessed ONLY from your primary OTP device, has no OTP to log in, and although limited in features, only requires a simple 5 digit pin, which is much easier to crack than an arbitrary length, alphanumeric + special char passphrase
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