Because FML is not registering new blocks's states to Block.BLOCK_STATE_IDS, ExtendedBlockStorage.get(int x, int y, int z) does not work, and therefor all get and set block methods fail with modded blocks.
I'll make a fork.
So far the 1.8 FML has been working pretty well, modeling is different and it is taking some time to figure out, and a few important mappings are missing (but that is an MCP thing). For the most part though, it seems pretty nice. Just looking forward to full Forge.