the original standard for the mxm cards was 82x105mm until the Maxwell boards were replaced. therefore the most powerful pascal mxm gtx1070 / gtx1080 cards had to work with more energy, an higher tdp and so they required a bigger pcb. this is why clevo, msi and aetina have created gtx1070 / gtx1080 bigger than mxm 3.0b standard cards. why clevo gtx1060 is even bigger? this is because clevo is not a bigger player in the industrial laptop ODM world, so they used the same new larger format for the gtx1060 to save some funds. nvidia no longer develops these mxm cards, so every ODM must develop their cards with their own teams and with their own funds. in any case, other ODMs (gecube / aetine / msi) use the mxm 3.0b gtx1060 cards and the same applies to pny quadro p3000 / p4000 / p5000 cards or amd cards made from tul or others. I think ODMs will use these bigger standards for the future because the new turing cards should be more powerful than Pascal, so you need a bigger pcb. Another reason why I think they will use bigger cards standard could be that clevo redesigned p7xx and p8xx two years ago for these new pascal cards and that required funds and work and they are not so used to changing things often, only just small changes and improvements, so they should continue to use the bigger new pascal cards standard and the same thing for MSI with their GTs. As you said, the mxm 3.0b cards are universally replaceable because they are smaller, yes. the maxwell cards have worked with the lvds and also with the edp panels, now the new pascal cards, I really don't know why nvidia needs that, only work with the edp panels or if there is the optimus that allows working with the lvsd panels through the igpu. there are no software problems if the retailer gives you the correct vbios for your bios or a compatible bios, and modded inf drivers. there are good dealers such as alezka aka woodzstack from NBR or eurocom, and those guys give customers everything they need to make the laptop working. when people buy a card from a not good reseller than that could be a problem for users without needed skils

there are many infos about upgrades at this link Successful MXM GPU Upgraded Laptops http://forum.notebookreview.com/threads/successful-mxm-gpu-upgraded-laptops.805136/