I moved to Pittsburgh in 2016 and started working downtown. I've ordered hundreds of times in the 7 years I've lived here, because it's the best Thai place I've had, for one particular reason: spice
I've eaten from so many Thai restaurants since I got here (that probably includes the one you're thinking of right now), and most places have a pretty mild "10", and even then the spice is just fine. At Siam Thai I usually hover around a 7 or an 8, and that's much hotter than most places top out at. But it's not just heat, it adds a depth to the flavor as well. I have no clue what they do differently, but it works big time.
They also tend to be very consistent, but I can tell when someone is filling in for the main cook. There was recently a 3 month span of subpar Siam Thai. Now keep in mind, Siam Thai off its game is still better than 90% of the Thai restaurants out there. But tonight they delivered one of the best meals I've had in a while. Everything was cooked so well, and the spice was HOT. Definitely a level or two higher than expected, but me and my wife were happy to enjoy the burn. Side note: Thai food is great if you're sensitive to gluten.
They're a small place of Smithfield downtown, and sometimes only a single person will be running it, and they're in the back cooking. They did indoor seating before the pandemic, but only pickup now. I'm also assuming they're family run, because sometimes they'll close for weeks/a month with a note left on the door.
TL;DR This is my favorite Thai in Pittsburgh, it's a small local business, and you should definitely eat here. And probably choose a spice level a little lower than you normally do.