Is this kind of craftsmanship actually acceptable for a professional deck build?

That is a massive red flag and you are defintely not being too picky. If the miter joints are already pulling apart before the crew has even packed up thier tools, it means they’re using wet, low-grade pressure-treated lumber and didn’t bother to compensate for the inevitable shrinkage. Or worse, they just can't cut a 45-degree angle to save their lives. A pro would usally **picture-frame** the deck or use butt joints if they know the wood is going to move, because miters on outdoor decks are notorious for opening up if not done perfectly with seasoned wood.

The splintering and uneven edges suggest they’re using a dull blade or just ripping through the material as fast as possible without any regard for the finish. You pay for a professional because you want those clean, consistent lines. If it looks like it was hacked together with a dull handsaw, then you’re not getting what you paid for. It’s the kind of work you’d expect from a DIYer’s first attempt, not a paid contractor. Those extra blocks around the vertical posts are the most concerning part. It sounds like they messed up the framing layout and are trying to **shim the posts** to make the railing line up. Posts shold be structurally integrated into the frame with proper carriage bolts or specialized structural screws, not held together with scrap blocks that are already water-logged. If they are already soaked through, they might be using scraps that were sitting in a puddle or just poor-quality wood that is going to rot out way faster than the rest of the deck. You need to stop them before they go any further. Tell them the joinery is unacceptable and the post mounting looks like a structural failure waiting to happen. Do not hand over the final check until this is ripped out and redone correctly. If this is how they handle the visible parts of the deck, I’d be genuinely worried about what the joists and ledger board look like underneath where you can't see them.

I'm about to start getting quotes for my own backyard and this is terrifying. I didn't realize those miter joints were so risky on a deck. When you talk about 'picture-framing' instead, is that somthing a beginner can easily spot in a contractor's portfolio? I want to make sure I hire someone who actually knows how to handle the wood movement. Also, is there a specific way to ask them what kind of moisture content their lumber has before they start? I’d rather sound like I know what I’m talking about so they don't try to pull a fast one on me.

I'd be calling the local building inspector imediately. If the joinery is this sloppy and they’re using mystery blocks for the posts, I’d bet money they didn’t pull a permit. Those extra blocks aren’t just a structural eyesore; they’re massive moisture traps. Once water gets caught between those scraps, you’re inviting rot to eat your frame from the inside out in just a few seasons.

Don't buy the 'it'll tighten up' excuse either. Pressure-treated wood only shrinks as it dries, so those gaps are going to look like canyons by next summer. If they can’t handle basic blocking, the ledger board is probably a disaster too.