I am a welder, and there is nothing wrong with those welds. Fitment is poor, but that's what you get for using pie-cuts from a straight piece of tubing against a cut in a mandrel bed radius. As long as he back purged or used Solar Flux the welds will be solid. The area doesn't look over heated, which is a plus. Most welders tend to overheat stainless trying to fill in gaps like those, so looks like the guy know what he is doing.
Uneven bead and misaligned... seriously? Can I see your welds? Those welds are decent, unless you are TFS Justin, Jodi, Kanekid, or Aaron-6160, then they are definitely subpar. Another good welder from this website would be
@GTI Jake
Look inside and see if the weld permeated - penetrated, welds don't permeate, unless you are referring to soldering. Which in this case, working on 16,14 or 11ga stainless, penetration is a given with the color of the welds. The concern here would be back purging or flux use to prevent sugaring... which will definitely result in cracking in a down pipe with those tight bends .
If you look at the welds, there is barely any deep purple, mostly silver to straw color, some pink-light blue, which is ok. Silver to straw is preferred, pink to purple is fine. Dark purple to any dull color is overheated, from those pics I only see 3 spots, and they are about 3-4 dabs long... for me there is no concern at all. A lot of companies would go over those welds with no filler on a decent pulse setting to cover them up, you will never know and you will pay them $$KK for their pretty colors. Or they will clean it up with Surfox or pickling paste, and run the tubing through Scotch-brite pads and remove all the color leaving a baby smooth surface.