Porting a game from CF2.5, I noticed a lack of certain features in the GM spriter SPI (though I may be missing something.)
Can't set animation without blend time (can only set blend to 0, which still causes 1-frame-long bugs when changing animations.)
Both APIs have an issue where you can't change the animation if there's blendtime (even if it's 0) if the previous animation is finished. I got around this in CF2.5 by checking the currently playing animation against the target animation and the spriter "animation has finished" event being true, and if so, changing the animation without blendtime and setting the animation time to 0 . I don't see any way to find the currently playing animation in GM's API, nor a way to set animation time, and as said previously, can't change the animation without blending.
I also don't see a way to set an animation to a keyframe, which I used extensively in CF2.5.