This is just a question. Tomato has an Adblock option under Advanced>Adblock. On that page, there is the option to create a customized Blacklist or use a pre-configured Blacklist URL (like Pi-Hole does).
What are the added benefits of using Pihole over the router's adblock capability, assuming the same blacklist of URLs?
I ask because, while I love Pihole, it does not seem to properly or easily route my VPN traffic through it (this solution, while marked solved, doesn't work anymore). So when I use my VPN, I lose my Pihole. Also, if my Pi crashes, my network is not available.
I can answer the second part - run two PiHoles in parallel. You can do this with a Pi zero for about $30 with power supply. I list one PiHole as the first DNS in the router and the other as the second. With my Apple router, 34K queries per day go to the primary and about 500 queries per day go to the secondary. That's with 26 clients. And, the router is the DHCP server, so either PiHole can fail and the other takes over immediately.
I run my own DNS resolver, so with VPN I need to use their DNS or I'll have a DNS leak expose my IP address.