actually without MCP mods would be a LOT harder,It all depends on how the mods are supposed to communicate with the base game. In that regard there is no difference between C++, Java, C#, Pascal or any other language you can think of.
Besides, If it weren't for Forge, mods would be a lot harder, even in java
remember that MCP is what decompiles and deobfuscates the java code, and decompiling java is much easier than decompiling C++