There are several aspects of this movie that are terrible.
The acting is on par with that which you might find in a play put on hastily by teenagers, who forget to make a script... or a plot.
The script, if one exists, is the kind of thing that makes the likes of 10,000 BC or Cats & Dogs look like masterworks. It mostly consists of one character telling everyone what is happening then another saying "oh yes, that is happening", to which a third character will exclaim that "I have also noticed it happening, and this is how I feel about it." Which may or may not be accompanied by an expression, which is supposed to convey an emotion but somehow doesn't.
The story was clearly not story boarded. That's the only conclusion I can come up with as to why giant swathes of the plot are skipped over from scene to scene. I have actually advised several film students to watch the movie as a warning, to illustrate what happens if you try and make a movie without properly planning it beforehand. You end up with missing scenes. In the case of Sinbad, you are missing at least 20 minutes, since the movie purports to be 89 minutes long but is actually only 69.
Editing. It's one of those jobs that is utterly thankless. If you do a good job, nobody notices but if you do a bad job, you ruin everything. As an illustration of what you can expect, we have a scene in the movie where, not once but twice, the camera has a slow, lingering shot of what we're told is a honey comb (people who have seen a honey comb know that they rarely look so much like a throbbing member though), the shot is accompanied by tense, combat style music. It is surreal. As if to compliment that, we have combat scenes, scored by gentle, Sunday strolling music.
Effects. Much has been made of the effects by the fake reviews but they really, really shouldn't. The effects are just plain bad. They are bad, to the extent, that the 1958 Sinbad movie, which featured the stunning work of Ray Harryhausen, is leagues ahead, not 50+ years behind. The idea of paying homage is fantastic and certainly stop-motion, with the modern tools available can really be brought to the next level, it just hasn't been here. It has dramatically missed the next level, fallen short and landed in a vat of angry alligators.
In short, the movie is a disaster, the likes of which most will be lucky to never see. For everyone else, watch this movie, only so that you might warn others.
Please insert your own sign-off pun, based on movie quality, containing the words 'sin' and 'bad'................................. ...............here.