If you are using lambda and suddenly need to run a million lambda you just can. If you want to have the ec2 support to run a million lambda at some time you are going to pay for a lot of sleeping computers.
If your spikes go 5x over your base load, EC2 is still cheaper. Even if your load spikes are 10-20x over your base load, EC2 can still be cheaper. Past that, lambda is your better bet. If you are comparing to Hetzner or OVHCloud, you those numbers are 25x (definitely cheaper) and 50-100x (possibly cheaper).
