It seems that CLEU SPELL_CAST_SUCCESS no longer occurs for spells with cast times - it only fires for instant cast spells now. I couldn't find anything about this change anywhere - has Blizzard said anything about it or why they did it? Either its a bug, or they did it for some reason of game/addon balance, but I don't really see how it could have been such a big issue to warrant its removal.
Perhaps, but UNIT_SPELLCAST_SUCCEEDED only fires for units with valid unitIDs. You can't track when someone in your general vicinity has cast a spell, unless there is a buff/debuff associated with the spell.
Overall, the loss in functionality isn't tremendous, but it still could cause some issues, especially with addons that are expecting it to function. Mainly wanted to create this thread just so that people searching for information about why their addon isn't working properly could have at least one relevant result. Not being able to find anything can be frustrating, to say the least.