Well, your tent is already going to come lined with a mylar type reflective material on the inside, it is going to be very modular so you can break it down or build it in less than 10 minutes. It is going to have a waterproof drip liner in the bottom to keep spills from going everywhere. They are durable and will last. My oldest one I have right now is about 5 years old and although I have a large dedicated grow room now, one of my tents is still right next to my grow room in case I need to isolate a problem plant. The money it will cost you to line the inside of your plywood with mylar will pay for a tent or come close to it.
As for the positives of a wooden grow box, you can disguise it to be very stealth. That is the only reason I can think of for building one. Well, I apologize, that and the fact that you can take pride in knowing you built it yourself.
Hey, it is your time and wood. I am not going to say don't do it. I just know from my perspective, after owning several tents as well as building a grow room from ground up, I would invest a few dollars in a tent for my first few grows. You will want to go bigger. I started with a 3x3x7 tent, wound up buying another one, then bought a 5x3x4 then built a 10x12x10 dedicated grow room. I first started out thinking the same thing as you, I would build something small to star with. Looked at the price of mylar and for what mylar would have cost to line the box I was going to build I bought a tent. I still own that tent and use it regularly.