One of the reasons would be that the tunnel sometimes can’t recover after a disconnect. An example would be the problem referenced in this post:
This can become especially painful when running n8n using a Wifi connection, but even with an ethernet network, network connections might get interrupted temporarily.
Also, keep in mind that this tunnel is a free service offered by n8n without any uptime guarantees. It might take a while to fix issues should they occur. Such problems would not typically occur when hosting n8n with a static IP with a public domain.