A (not very) small description of why the new system can be (and often is) dangerous.
V: Ramp
#: Wall
~: Magma
.: Floor
D: Dorf
_: Open space
########
#~~~D..#
#~~~V___
#~~~....
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This is a portrayal of the turn immediately after completing the new channel (the open space having been dug way earlier and cleared to prevent accidental meltings, fps issues from blinking tiles, and headache-inducing issues from same)
Now I'm aware that likely no one would actually make a setup like this, but I just wanted an obvious example of 'dwarf moves to other side to dig, placing him on the opposite side pathing will take him if it can find a route'. This can usually be avoided by being familiar with what side a dwarf prefers to build on. Easy for those that have played for a long time, but for newer players, or people who haven't played in awhile, or heck, just for people not thinking about it, this requires either trial and error, or mining the wiki.
In this particular case, at a good fps and moderately high gps (if I'm remembering that seting right), the dwarf in question will immediately path back down the ramp regardless of the impending doom, especially if they're particularly agile and the liquid hasn't even started flowing yet. In this case MAGMA. Dwarf then gets his feet roasted, catches on fire, and most likely dies in the heat, with a decent chance of losing the pick, and whatever else said dwarf happened to be carrying.
Even better, let's say the dwarf is on fire, and another dwarf is nearby, and there's a hospital bed open. Urist McHelperDorf hauls the still-flaming miner, catching on fire himself, and merrily begins setting your fortress on fire. Let's say for FUN(tm)'s sake, you haven't forbid your dead, or their items in the o menu! Urist McGraveRobber decides that those hot pants are really styling! Why, they seem to be on fire! He then dons pants, and goes to the booze stockpile for a drink. So with the tantrum spiral and fire spreading like ebola's bastard child with AIDS, the tiny flashing red triangle is insult to injury.
Now, this might seem somewhat melodramatic or exaggerated but so help me Armok, there will be like ten posts on the facepalm thread for this exact reason.
And, before someone says 'but you can avoid that with careful painstaking micromanagement!' allow me to list the ways this can be prevented to my knowledge.
1) Carefully monitoring the breach, pausing when the miner arrives, . your way to the completion of the ramp, then designate and forbid the traffic for the new tile. Provided the pathing cost is now above what going around would be, Urist McMiner will (hopefully) ignore it, and a helpful mason can patch it with a floor, or an out of place pillar with a foundation of liquid rock.
2) Dig the breach area in such a way that a dwarf would have no reason to path over the ramps created, though depending on the situation a magma man might delight in running up the convenient ramp and mauling your miner anyway. This requires more careful planning and in some cases, less attractive breach zones. Which still have a lovely blinking triangle combining with every other moving flashy shiny thing to slowly chip away at your fps and processor.
3) Use either a dwarf powered pump (with grate, preferably, to prevent carp-based doom), or a powered pump. This causes its own problems, such as overflow, splash, and other such fun things that are an even bigger problem with magma, and woe be unto you if you can't safely floor and grate the area, lest beasties drag your dwarves into the water. Please note that the pump would require the creation of a power farm should you desire a multi-tile wide breach (for carrying magma long distances), and even otherwise, require the additional steps of constructing the apparatus for the pumps in question, the machinery involved, and the safety measures, on top of the careful planning. Still, with that lovely blinking triangle taunting you, with it's "Blah blah blah, look at how I can make your dwarvenly lives hell! The triangle reigns supreme!" Mocking you!