A little old, but many of these "VPNs" are still probably insecure... For the record, from August 2016:

Millions of users worldwide resort to mobile VPN clients to either circumvent censorship or to access geo-blocked con-tent, and more generally for privacy and security purposes. In practice, however, users have little if any guarantees about the corresponding security and privacy settings, and perhaps no practical knowledge about the entities accessing their mobile traffic. In  this  paper  we  provide  a  first comprehensive  analysis of 283 Android apps that use the Android VPN permission, which we extracted from a corpus of more than 1.4 million apps  on  the  Google  Play  store.

We  perform  a  number  of passive and active measurements designed to investigate a wide range of security and privacy features and to study the behavior of each VPN-based app.  Our analysis includes investigation of possible malware presence, third-party library embedding, and traffic manipulation, as well as gauging user perception of the security and privacy of such apps. Our experiments reveal several instances of VPN apps that expose users to serious privacy and security vulnerabilities, such as use of insecure VPN tunneling protocols, as well as IPv6 and DNS traffic leakage.   We also report on a number of apps actively  performing  TLS  interception.  Of  particular  concern are instances of apps that inject JavaScript programs for tracking, advertising, and for redirecting e-commerce traffic to external partners.

16 page pdf: https://research.csiro.au/ng/wp-content/uploads/sites/106/2016/08/paper-1.pd... https://dl.acm.org/citation.cfm?doid=2987443.2987471