Shadow is one of my personal favourite 'bad' cards.
Firstly, why do I like it? As an early game ice rez, it typically fires its subs and so the Corp gets 2 back (for a net rez cost of 1), and then the runner will have to pay a couple of credits on the trace or clearing a tag shortly afterwards. Overall, a nice credit swing result from rezzing ice. Also, once rezzed on a central server or taxing 'asset' remote, a runner will rarely make a normal run through it before getting a killer out to deal with it. And even once the runner has a killer out, they're probably spending 2 to break this every run, regardless of what killer it is. Also, on occasion a runner will get careless and run through a Shadow on last click without a killer, letting you land a killing tag. (But sadly, this scenario is extremely rare).
However, Shadow has one really big problem. If the runner wants to make a high-value run, such as The Maker's Eye, Legwork, Wanton Destruction or Account Siphon, they'll often be happy to just run right through Shadow. Also, once the runner has a killer installed, the net rez cost is the full 3 which is not great for the small tax it provides.
Advancing Shadow is rarely worthwhile. The most likely scenario of triple advancing it to strength 4 to mess with Mimic is still hard to justify - the Corp has spent a hefty 3 clicks and 3 credits, now the runner can still just run through it and clear the tag afterwards if they really need to.
There is one ID where Shadow performs clearly best, and that is Tennin Institute. With a tiny 1 inf cost, it is affordable advancable ice along with Ice Wall to support Trick of Light. Also. its low net rez cost is perfect for setting up early game defenses, and ongoing tax is good for grinding down the runner.