I haven't delved into OC'ing my Nexus but from what I've read it seems you can't set the CPU to 1.5 because the MDU DPLL must be set at double the desired speed.
Aka 2Ghz for a desired 1Ghz or 2.4GHz for the stock 1.2GHz.
Now apparently when the DMU DPLL is set to 3Ghz to achieve 1.5 that's when people are running into problems.
Having said that, people have jumped past 1.5 onto 1.6 and seen some decent stability, but there appears to be a bug that prevents the CPU from running stable at 1.5, which is strange considering the 4460 is supposed to be stock 1.5.
I used to overclock with EVGA's team and a lot of times we would find that certain versions of CPU's wouldn't run at certain speeds no matter how much we adjusted the voltages/settings. But if we bumped the clock up even a few Mhz higher, it would run fine. So sometimes I don't know that there are actual answers to why you can't reach a certain speed, but I know it's happened to us on dual and quad core overclock tests.
Give it some time, I think you'll see some kernels running at 1.6+, but for now until they find out why 1.5 isn't working I think there are a couple kernels for 1.4GHz.
I can't even bring myself to over clock my 1.4ghz quad core laptop that has cores designed for 2.3 ghz because every time I search for instructions, it's people saying not to do it on a laptop because it doesn't have as much of a cooling system as a PC. And here you guys are overclocking a phone that runs hot stock... in a method that voids the warranty. I don't know how you do it.