... the initial induction chemo they may do well.

Biggest danger during initial chemo is tumor lysis syndrome. What does chemo do? It destroys any cell that tries to divide while the chemo dose is on board. Since cancer cells divide so much faster than healthy cells, most of the cells that get killed by the dose (percentage-wise) are cancer cells.

In a newly diagnosed patient with a quickly-multiplying type of leukemia, the tumor burden (i.e. sheer number of cells) can be very large. When all those cells break up at once, their internal contents are disgorged into the blood stream. Chemically, that means lots of electrolytes and things like uric acid. This can totally screw up the blood chemistry and damage organs in any number of ways. Fluid balance can be affected too, so you get things like pulmonary edema.

Additionally, the over-multiplying white cells are eating up resources which should be devoted to production of normal cells. So you can get all sorts of blood clotting problems, both too much (DIC) and too little (low platelet counts) -- hence brain bleeds, etc.

Gotta do the chemo though if you are interested in living. There generally isn't too much time to wait around. If you can make it through the first hit and reduce that tumor burden, tumor lysis won't be such a problem for the second round.