I have the Vodafone router and have found it to be quite stable. BUT I don't use it for anything other than modem. It feeds into a second Unbiquiti EdgeMax router that acts as the real brain for my network.
A smaller number of devices does seem a reasonable suggestion given my experience.
I have decided that this Vodafone router is absolute garbage.
Theres a huge thread on the vodafone support forum about random restarts, which has now started happening to me.
The solution is 'have less devices connected'.