If PiHole is malicious, there is already an attacker on your network, DNS Spoofing is just one example of the possible consequences. The PiHole can also port scan, connect to services etc. I don't think mounting an effective phishing attack on a user would be very hard.
My point is that both scenarios are catastrophic, and its hard to justify choosing one over the other on the grounds "the developer might be malicious". Telling people "don't worry a DNS server can't do much" is massively understating the problem, considering all the local network devices directly exposed to the PiHole device and the fact it is the DNS server.
As I said, I use both and cross my fingers that Mozilla / Open Source code review / the GDPR mitigates the risk of a bad developer