I think there's like 4 raid difficulties now. It's admittedly become a bit convoluted
No more convoluted than it's been before. From Wrath up until Dragon Soul you had 10-man normal, 10-man heroic, 25-man normal, and 25-man heroic. 4 difficulties. Prior to that in Ulduar you had 10-man normal and 25-man normal, plus 10-man HC and 25-man HC. 4 difficulties. Prior to that it was classified raid-by-raid, e.g. Black Temple was a 25-man raid while Karazhan was a 10-man, with no extra heroic difficulty or HC difficulty like Ulduar had. I would argue that the way TBC handled it was more convoluted than the way Warlords has it. The Warlords system is simple: the difficulty scales as per the raid size in both normal and heroic, while mythic is a fixed 20-man to allow for the tightest possible tuning of the fights, in addition to LFR (4 difficulties in total). Much better and more manageable than starting out in a 10-man raid, then moving on to a 15-man, then on to 2 25-mans, all while praying that nobody on your raid roster was poached away or left (and praying for an adequately experienced and geared replacement for when they inevitably did).
In terms of sheer number of difficulty modes, Mists was the most "convoluted", and even then one has to be a bit of a simpleton to not be able to make sense of it. There's LFR, then Flex which scales depending on raid size, then normal and heroic (with heroic since renamed to mythic), which adds up to 6 difficulties in total.