'Specified key was too long; max key length is 767 bytes' on SobiPro installation (some servers only)
767 bytes is the stated prefix limitation for InnoDB tables in MySQL version 5.6 (and prior versions). It's 1,000 bytes long for MyISAM tables. In MySQL version 5.7 and upwards this limit has been increased to 3072 bytes.
The index limit is 767 bytes, not characters. And since Mysql's utf8mb4 character set (which the rest of the world calls utf8) needs (at most) 4 bytes per character you can only index up to VARCHAR(191)