The GOOD:
Being able to place your units on the enemy side of the field opens great and exciting new tactics.
The BAD:
The current mechanics are very underwhelming for a legendary. Currently the weakest legendary in the game, even worse than Templar and Orc Slayer.
Very finikey and hard to use with very little rewards. Warriors get placed down and they just start attacking. It's very hard to use a Warrior incorrectly. For spy, it feels like I need an advanced degree in physics to predict how these will even path. Even if the spy survives to do it's party switch move in PVP, it will likely get itself and its new convert killed given how strong "mass armies" are.
The dude dies to AoE damage, well before it gets out of stealth. Hell, my first captain spy died instantly at battle start because the stealth didn't trigger.
So what to do?
1.) Give Spy invincibility for the duration of its stealth to prevent it from dying a miserable death not befitting a legendary.
2.) Provide the converted new ally unit AND the spy a 5 second invincibility once the convert.
3.) If there is no priority for the team swap, make the priority for converting to be Melee > Ranged > Closest.
4.) Make the SPY market follow STRICT placements requirements. The last thing I need is for a rogue to be placed with all the spys, they all move together, and then a barbarian kills the rogue and the 30 other spys next to it.
5.) Add technology to the game that will allow your spy to target a specific unit in the game for it to sabotage first, or be able to change its priority tree flexibly. Sometimes I want the Spy to gimp an arty. Sometimes I want the spy to gimp the captain paladin. SOMETIMES I want the spy to gimp that one vengeful warrior with the flag soul.
Make the spy EASY to use with consistent results that err on the side of strong, or make the spy HARD to use and can be anywhere from mediocre to absolutely game changing Stream Clip worthy moments.