In principle, roaming SIMs that give access to all 2G, 3G and 4G networks in any one country sound like a great solution to the limitations of making a cellular internet connection through a single network operator (e.g. Vodafone).
Finite coverage and potential intermittent congestion are the key issues that can cause downtime if a single network SIM is used as opposed to a roaming one. However, using a roaming SIM is only part of the solution.
Network selection is a function of the hardware NOT the SIM card so whilst a roaming SIM seems like a good idea in principle, it is possible to use a multi-network SIM but still find that your device (router/modem) does not move to an alternative network when connectivity is lost.
The full details of the process that governs ‘network selection’ is described in 3GPP standard TS 3GPP 23.122 which can be downloaded from ‘portal.3gpp.org’.
This will help explain to the more technical reader the full detail and limitations of automatic network selection.
The Author’s anecdotal experience over many years is that system integrators opt for a roaming SIM and are sometimes disappointed to discover that they can still be confronted by obscure connectivity issues that are hard to understand and more importantly, hard to fix.