Good question and I can see why initially you might be skeptical. However fully invincible moves are not as common as you think in this game and DS is the only character with 2 invincible wake ups that auto correct into each other. This blows up so many crossover attempts and completely nullifies reverse wake up setups. If I land a sword flip yes I only get 8% but I also get a hard knockdown. Which is an invaluable trait in fighting games and immediately reverses momentum in deathstrokes favor.
For instance if I wake up flip against say catwoman. I can dash up and time my 50/50 to stuff her wake up so she has to guess in order to get up, if she's right I'm still safe, if she's wrong she's eating more damage into the same scenario. Even if they have wake ups that can't be stuffed sword flip immediately lets me reset spacing, start zoning if I wish, bait one of their unsafe wake ups, go for a crossover etc. It's 8% damage but the momentum shift is huge, especially for a character like Deathstroke who can threaten his opponent from basically all ranges.
Sword spin can be neutral jumped yes, but that just puts me at the second phase of the meta and if I read neutral jump I can respond with a d2, backdash into whiff punish, sword flip, etc. Sword flip being multihitting and having a lengthy number of active frames is also huge because it allows me to hit confirm into MB Sword spin. If I see spin hit a Mb it-- dash -- sword flip, for somewhere around 20%, a hard knockdown, and a huge momentum shift.
So let's say you don't want to deal with the wake up threats and decide to just let me get up? Well that means that the threat alone of my wake ups has caused my opponent to let me get up for free, which is a huge advantage and really all you can possibly ask for in a wake up attack.
He has the best wake up tandem in the game imo.