if tsunade had SM she would have used it, against madara at least + she didnt becuase she didnt have it whilst not being hokage n then wen she became hokage she was too busy to train her self
i dont think the shikkotsurin has much to do with it, i think its like a one shot thing, for eg. if pervy sage absorb too many chackra, he turned into a frog statue, may be with slug mode if tsunade absorb too many nature energy her body disintegrate, because through out the anime katsuyu is good at healing and splitting apart just fine. plus kabuto mention that each sage mode had a weakness.
have you ever wonder and thought that, thats the reason why she still lives, she is a master of slugs, and slugs can split apart just fine. hence why katsuyu just said i can stitched you back up and she said, you must help the others first. maybe slug mode have some huge risk especially since she hasn't mastered it.