I'm quite ok with Book's ship getting a spore drive. Given that they mentioned that the next generation Spore Drive was already in R&D at the start of the season, I can see Spore Drive's getting mass produced and spread across StarFleet.- I hope the writers thought through the implications of giving Book's ship a spore drive. I'm hoping this is temporary and the drive is removed or destroyed by the end of the arc.
I was surprised at how compact the tech had become, it's literally not much larger than a mini oven.