Why is this an issue?

Note that this rule is very opinionated and mostly used on legacy or safety-critical projects.

Some reasons for enforcing this rule:

• If two clocked processes write to different fields of the same record signal → many simulators / synthesizers see it as multiple drivers on the whole record → 'U' or synthesis error. Very easy to do by accident in large designs.
• Records tempt people to group unrelated signals. It reduces readability, makes slicing / renaming / searching harder, encourages “god records” that are passed everywhere.
• Older Quartus, older Vivado, some Synplify versions had nasty bugs with records: Wrong multiple driver errors, incorrect flattening, bit misconnection, AXI record problems, unexpected 'U' propagation.
• Some old tools flatten records differently than individual signals. It can create slightly different timing / area / routing → non-deterministic results when refactoring.