/etc/network/interfaces does not allow for multiple connections (easily), which can make roaming more challenging.
See [HOWTO] OSMC/Rasp Pi as OpenVPN client - #35 by sam_nazarko for more about VPN, ConnMan and /etc/network/interfaces. The only issues with ConnMan seem to be raised by VPN users. They would be better served working with us to get VPN working with ConnMan, then everyone wins 