Mockingbird copies exactly what was printed on the original creature, except it's a Bird in addition to its other types and it has flying, and nothing else (unless that creature is copying something else or is a token
For example, let's look at Zan Syed's winning decklist from the SCG Classic
Without these safeguards in place, older cards can often become huge problems for modern competitive play, especially because newer cards will have to abide by those restrictions a lot more consistently
If that deck has three or more Malamar in play, and enough Energy and a Necrozma-GX in the discard, each of their Marshadow-GX become a threat
Cons : Lower MUR chase compared to Inferno X
Stolen Uniform Stolen Uniform is like blues Act of Treason for equipment, except its an instant, so you can do a lot of trickery