A couple of years ago I started with Atmel SAM devices and I meet ASF technology. I didn't like it at first, and I don't like it now that is named ASFv3. I don't use full-ASF code in my production code.
Anyway I used and studied ASFv3 code generated by Atmel Studio, because many details aren't well descripted in the datasheet. At least I'm able to understand the correct configuration and meaning of peripheral registers. Moreover, there are many errata and workaround that are available in ASFv3 code that I wouldn't ever implemented myself in the correct way.
For example, it seems DFLL configuration on SAMD20 is tricky, because some registers are different on different chip revision (and these differences aren't documented) and some workarounds are needed because of some errata on DFLL. For example see _system_clock_source_dfll_set_config_errata_9905 and system_clock_source_dfll_set_config here.
After studying ASF code, I copy and paste only the interesting parts. I don't need all the complexity of ASFv3.
Now I meet Atmel START and ASFv4. I tried to configured DFLL on SAMD20 and the generated code doesn't include any chip revision differences in registers. So what happens? It seems ASFv4 is lighter and more readable, but I don't know if it is a mature product or not. Maybe all the code in ASFv3 was useless?
My idea is to stay with the latest version of ASFv3, but I don't know if it will be thrown away in the immediate future, in favor of ASFv4. Do you have more info regarding the future of ASFv3? Wil it be maintaned by Microchip? Will be errors fixed?