There're several mixed FM/MFM floppy formats out there...
What's really fun are the TRS-80 Model I/III dual-boot disks where there is mixed-density FM and MFM
in the same track.
Anyone knows which hardware (I mean FDC controllers) + software combination works good enough to be able to read and write them?
Catweasel can read and write these formats (even the TRS-80 Model I/III dual-boot ones) as long as an image file format that recognizes mixed-density, such as DMK, is used.
Since you didn't specify PC-only (although that is probably what you meant), a TRS-80 Model III or 4, especially the non-gate-array Model 4, can read and write these mixed-density formats just fine, but only at the 'low-density' data rate. For PCs with regular FDCs the choices are more limited, and to the best of my knowledge a 765-style FDC can't do the mixed-density
within a single track format prior to the write. You have to have special creator programs even on the TRS-80's to make copies of these disks since the techniques used to get a mixed-density track are specific to the particular disk (how many sectors FM versus MFM; many if not most of these dual-boot disks have a single block of one to a few MFM sectors for the Model III boot with the rest of the sectors on track 0 being FM and all other tracks being FM as well).
EDIT: The FM/MFM mixed in one track format is possible on the TRS-80's because the WD1793 controller has two specific features in its feature set: variable post-index gap, and a Force Interrupt command. These two features together make it possible to write a track in one format with a long post-index gap, then overlay a write track in the other format, then issue a Force Interrupt before the write track gets to the end of the first format's post-index gap. It's partly because the write track on the WD179x controllers requires the programmer to write out pretty much every byte of a whole track that this works at all.
I'm not sure how mature the write support of the Kryoflux is; I have a Kryoflux but haven't had opportunity to put it to much use as yet; my two Catweasels (one Mk3 and one Mk4) are my workhorses when I have needed to read and write odd formats in the past. I use the cw2dmk tool available from Tim Mann's website.
FM support in typical PC 765-style FDCs is very spotty; Dunfield's site and ImageDisk utilities can help you find out if your specific controller has the support.