It really comes down to how many sites you block. It comes with some default blocklists, but wally3k hosts a site tells you the status of different blocklists: https://firebog.net/
As for what is seen when a site fails - well, it could vary. If a JS file fails to load, the site will either look bad or not at all. ANd yeah, some apps may have issues, but pihole comes wiht a built in query log so you can see which devices just made which DNS queries, along with an easy-to-click whitelist button.
There are different blocking modes you can choose from that return different response codes, so you can choose what works best for your environment: https://docs.pi-hole.net/ftldns/blockingmode/
I would recommend checking out the pihole discourse(https://discourse.pi-hole.net/) or subreddit. The community is very active
As for what is seen when a site fails - well, it could vary. If a JS file fails to load, the site will either look bad or not at all. ANd yeah, some apps may have issues, but pihole comes wiht a built in query log so you can see which devices just made which DNS queries, along with an easy-to-click whitelist button.
There are different blocking modes you can choose from that return different response codes, so you can choose what works best for your environment: https://docs.pi-hole.net/ftldns/blockingmode/
I would recommend checking out the pihole discourse(https://discourse.pi-hole.net/) or subreddit. The community is very active