The main concern with smartphones are them being the weakest link in a chain, from a security and attack vector perspective.
It is known that many smartphones may have backdoors built into them which allow for monitoring/data mining/state surveillance. Here is an example of this in action:
Pre-installed Backdoor On 700 Million Android Phones Sending Users’ Data To ChinaWASHINGTON — For about $50, you can get a smartphone with a high-definition display, fast data service and, according to security contractors, a secret feature: a backdoor that sends all your text messages to China every 72 hours.
Security contractors recently discovered preinstalled software in some Android phones that monitors where users go, whom they talk to and what they write in text messages. The American authorities say it is not clear whether this represents secretive data mining for advertising purposes or a Chinese government effort to collect intelligence.
International customers and users of disposable or prepaid phones are the people most affected by the software. But the scope is unclear. The Chinese company that wrote the software, Shanghai Adups Technology Company, says its code runs on more than 700 million phones, cars and other smart devices. One American phone manufacturer, BLU Products, said that 120,000 of its phones had been affected and that it had updated the software to eliminate the feature.
https://www.nytimes.com/2016/11/16/us/politics/china-phones-software-security.html?mcubz=3 Here's another example of smartphones not being the most secure platform:
An Obscure App Flaw Creates Backdoors In Millions of SmartphonesFOR HACKERS, SCANNING for an open “port”—a responsive, potentially vulnerable internet connection on a would-be victim's machine—has long been one of the most basic ways to gain a foothold in a target company or agency. As it turns out, thanks to a few popular but rarely studied apps, plenty of smartphones have open ports, too. And those little-considered connections can just as easily give hackers access to tens of millions of Android devices.
A group of researchers from the University of Michigan identified hundreds of applications in Google Play that perform an unexpected trick: By essentially turning a phone into a server, they allow the owner to connect to that phone directly from their PC, just as they would to a web site or another internet service. But dozens of these apps leave open insecure ports on those smartphones. That could allow attackers to steal data, including contacts or photos, or even to install malware.
"Android has inherited this open port functionality from traditional computers, and many applications use open ports in a way that poses vulnerabilities," says Yunhan Jia, one of the Michigan researchers who reported their findings at the IEEE European Symposium on Security and Privacy. "If one of these vulnerable open port apps is installed, your phone can be fully taken control of by attackers."
https://www.wired.com/2017/04/obscure-app-flaw-creates-backdoors-millions-smartphones/ There are other potential issues with smartphones in financial transactions.
There's a push for lower smartphone encryption standards by intelligence agencies. There's the potential for cellphone signals being wirelessly intercepted and decoded to mine passwords and other personal data. As usage of cellphone based payment and monetary transactions increase, the number of viruses and malware which target cellphone platforms are also likely to propagate.
Smartphones are very convenient. Whether they can realistically be secured is another question.