So, I can get any shards via Special Spins even though all of my opponents in the Arena I faced didn't have any?
Special spin rares aren't based on the decks you've faced to get to them. They just have four random rares, there's no correlation between decks you've faced and special spins.
If so, why is Shard of Patience the hardest Shard to get? Or is it just Murphy's Law?
There is no "hardest" rare to get, it's all just probability and RNG in deciding which rares are in which slots. The only rare that's significantly harder to get is a nymph, but that's obvious.
For SPECIAL SPINS only there is no hardest rare to get, except of course nymphs because you get relics instead (also relics don't show up in special spins)
For general spins there are harder rares to get because they're based on your opponents deck
Id say the least commonly used rares are: Trident, Morning Star, SoI, Pulveriser, and then the others are all relatively close to one another in how hard to obtain (note this is when vsing player created decks, not AI0-5)