How many Monero block explorers are currently known to exist?
What unique features separate one block explorer from another? What differences do they have in terms of privacy and security vulnerabilities for those who use and rely upon them?
There are (an unknown number of) other block explorers, but with less unique feature support as those mentioned above.
Note that when looking at the BCN CryptoNote chain on ChainRadar or Minergate, there is strong evidence that transaction volume is artificially created by the same insiders who faked the BCN inception date. Because MinerGate seems to be controlled by the same people who control BCN (plus CoinTelegraph, Changelly, and more) its blockexplorer should never be relied upon as your only source of data.
In addition to MoneroBlocks, ChainRadar and the Onion Monero Blockchain Explorer description from 2quick 4u, there is another new explorer worth mentioning:
Monero Explorer offers: