When it comes to accessibility, it is best to think of tables for just presenting data. You should avoid using tables to help with layouts, avoid merging and splitting cells, and always use a header row.
When tables are designed without these things in mind they can be incredibly challenging for those on screen reading technology or those who navigate a document or website using a keyboard alone.
Keep tables simple
Tables need to be designed in the most simple way possible to avoid challenges or barriers.
This is important because:
if a cells in a table are merged or split a screen reader struggles to navigate it and provide helpful information
tables within tables (nested) are difficult to tab through logically
screen readers use the column headers in tables to help read out the context for each columns and their rows