OpenUTAU is an Open source (aka free/libre software) and cross platform UTAU successor compatible with UTAU resamplers, other engines, including DiffSinger, and ENUNU.
There are many free software UTAU resamplers, most of then are based on WORLD OpenUTAU also has its own builtin resampler called WORLDLINE-R, older resamplers similar to WORLDLINE-R include vConnect-STAND.
DiffSinger is currently only supported using OpenUTAU, there are no plans to support QTAU.
OpenUTAU currently supports NNSVS, but there are plans to support NNSVS also in QTAU, theoretically supporting many other languages with the eSpeak-NG backend.