I'm in the middle of switching to Cypress mulch blend from Lowes. I like the Coco Coir, packs nice, good water retention, but it was very messy. It's not as bad with my Sulcata but My Hermanns really like to dig and even with a 12x12 slate to cross they still covered their food in it.
I currently have my Russians on all cypress. I like that it's easy to clean and doesn't stick all over them (too much, i dont like when it gets in the spaces by their necks and betweeb the body and legs) and can be taken out of the water easily. However I don't like how some pieces are sticking to the wet greens. I found 3 pieces of cypress in one poop and while it is passing through it still scares me. I'm thinking about adding coir to the mix, which I used to have. I just hate how it sticks to the food, them, and gets in the water. But there would probably be less of an impaction risk...
I use the coir (with sand) in all my enclosures (except for the big guys ). Any type of substrate can be digested by your tortoise, either accidently or on purpose. They will normally all pass thru their system, but sometimes they do cause problems. Sharp items may poke intestines, some substrates are more prone to cause blockage. Usually blockages do not happen if the animal is healthy, active, eating good and properly hydrated.
I prefer the coir/sand for several reasons. For one, I dislike dragging home those usually dirty and wet bags of mulch or soil. The coir comes in those nice, easy to carry blocks. I like that if I get my coir too wet, it dries pretty easily. It holds moisture easy and is easy to tell when it starts to dry out. The tortoises seem to like it for burrowing into. I like that it is soft, not sharp points like the cypress.
I do use some cypress for small areas in the enclosures, to make it a different type of substrate for the tortoises to use if desired.