Here is a list of the bad targets for Replicant and reasons why:
MSM/QSD devices are known to have bad hardware design where the modem controls GPS, audio (including mic), RAM, NAND. However, some MSM/QSD SoCs don't ship with a modem embedded, so they may be good targets.
Nvidia Tegra 2 devices are known to be too slow with Replicant, since there is no NEON nor any free hwcomposer/gralloc. Perhaps newer Tegra SoCs (which include NEON) would be faster on Replicant.
Motorola devices manufactured after the Milestone (inclusive). These have non-free locked bootloaders that check kernel signatures. sbf_flash, a utility capable of flashing Motorola devices, is non-free.
Free gralloc, hwcomposer, non-free 3d, beware: ultra high resolution(could be slow)
N/A (Has No Modem)
BCM4751: secret protocol
CM 10.1 (Android 4.2)
Qualcomm Snapdragon S4 Pro APQ8064
Really BAD: has too much proprietary things and the hardware is bad for freedom: modem transport is over HSIC (high speed USB), but it controls the GPS (included in the modem chip), audio and perhaps NAND too
CM 7.2 (Android 2.3)
Recovery, in-system recovery flash
Bootloader is locked
Kernel is signed, making a Replicant port impossible
both 8192cu and rtl8188eu WiFi chips need firmware blob
Some have rtl8192cu WiFi, some have rtl8188eu
More focused on running GNU/Linux than Android. Wiki
There is a small number of Do-it-yourself phones where the user is expected to assemble his phone. Examples of this include the odroid phones with comes with android support. However the issue is that not all users wants or are able to assemble their own phones.