I've been doing some investigating with this myself.
I think the new channelling system is less like digging a channel and more like digging a
trench.The only differences between building a ramp and digging a trench are:
A: The former is done from below, the latter is done from above.
B: You can use it to remove floor tiles that have no earth below them.
C: When digging a trench, the miner seems to avoid the trench, but when digging a ramp the miner is more than happy to stand in the trench and dig. Other dwarfs will have no issues stepping in the trench however, and once the miner is done digging, neither will he.
I actually lost a miner to C. And not in the typical "frozen in an aquifer" way.
I'd decided to build my fort into the side of a cliff you see. I'd started building from above, but I'd managed to dig out a room a little too close to the aquifer, and suddenly a every room below it was flooded. It was an inconvenience, but I worked out that what I'd do is I'd go to the room above, and mine out to the cliff face:
Then I figured what I'd do is I'd dig out a channel so that the water in the room below would be funnelled out, giving me time to get a wall up and block off the aquifer, so that I could then drain the lower levels:
It was then that I discovered that I was not digging channels, but ramps. I was initially pretty confused, but it wasn't too big a deal. After all, the water was still draining, and everything was working as intended, more or less:
That was, until my miner decided that the fastest way to get back to the stockpiles was
through the channel.