@simonkabz and @ leona could be most resourceful here
the only bus i used to know of that crosses to Ukunda is 2mk na ni mkebe
most buses terminate @island then you take a taxi to south coast or cross ferry halafu south coast matatus (not recommended the Likoni termini both sides is a small Nyamakima / Machakos
Pollmans and tourist shuttles yes.
How come nobody is mentioning my favourite bus MASH

solid reliable and have the best / safest receptions / waiting areas especially for newbies and those with cargo and kids
Trouble with buses is you dont know which one shall be allocated unless you are a keen traveller and you grill the booking guys mzuri
Took a ride on Modern coast having admired some on the road . . . Bill ya chiropractor nayo

they do have some very only buses chunga sana.
Those mboyz talking ill of Chania siyui Channia

wacha one of these days you find all the others full booked for 1 week. Utajua 2NK pia huenda pwani
Simba pia iko plying cross-country. Keep your options open and book in time
Also try the train and report back
you could also carpool and drive you get to enjoy better
"😖😡KQ makes money for everyone except the shareholder 😏😏 " overheard in Wazua