> The cost to run these for me is less than the cost of the cheapest vps (my total requests per month stay under the free tier limit).

I don't think this is a valid argument. Free-tier VPS do exist also.

On the other hand, if you don't trust unattended-upgrades [0], and prefer to spend time poking package manager manually (while at the same time considering that time an expense) - sure, that's a strong argument in favour of using lambda.

[0] https://ubuntu.com/server/docs/how-to/software/automatic-upd...