As some others mentioned, when the DNS goes down (which pihole is) your whole network is down. With the fragility (and slowness) of the PI, it’d be more likely it will go down, sooner than later.

Considering the cost, a good alternative, imho, would be some sort of thin client, with an energy efficient CPU. So, instead of getting 2-3 PIs, better get one of these TCs, while keeping your PI as a DNS backup solution.