I'm going to express my opinion... take it or leave it.
There's nothing wrong with using UCEProtect Level 1, but to use Level 2 or 3 is completely irresponsible and illogical.
For an IP Address to be on Level 1 the need to have actually sent spam. Nothing wrong with that.
Level 2 blocks large blocks of IP Addresses if there happens to be a spammer withing that block of IP Addresses. So they block servers that have never sent any spam but UCEProtect considers them guilty by association. UCEProtect warns that Level 2 should only be used for very strict spam filtering and that you will block valid messages if you use this level.
Level 3 is insanity. UCEProtect even WARNS you that this is for "Draconic" spam filtering and "NOTE: By using Level 3 for blocking, be prepared to lose some required mails too. DO NOT BLAME US, YOU HAVE BEEN FOREWARNED!".
UCEProtect is misused so much that many accuse them of nothing short of extortion. If you happen to have a server inside a data center and the entire ASN is blocked by Level 3 (which is very common) you must pay to have your IP Address whitelisted.
I am all for blocking spammers, but Level 2 and Level 3 block many valid servers. Those levels have nothing to do with specific spammers and using them you are contributing to the UCEProtect 'extortion'.