As noted in the comments of Use registers. by @alan_r_cam, partial registers are a seducing feature. It was born in the 70s when microprocessors were 8-bits wide internally and made 16-bits registers out of pairs of physical byte-wide storage. 16-bits operations would take two cycles instead of one.

The MC68000 was 16-bits wide internally and emulated a 32-bits architecture. So handling 16 bits quantities was not a problem.

But this practice has vanished in the 80s. Speed is now really critical and whole registers are stored and handled in a single cycle. The last processors that still implement this feature are the x86 line, using "µops" to decompose the partial accesses into discrete shifts and masks. This causes all sorts complicated issues, for example the detection of pipeline hazards and all kinds of dependencies.

Avoid partial register writes. They complicate the pipeline, add latency because you insert a shifter in the critical datapath, which slows the whole CPU down. Use explicit shifts instead. The YASEP has a dedicated unit for alignment and sign extension, the IE unit (for Insert/Extract).