Actually, it can be really dangerous to be a spy. You have to be right in the middle of the horde to accomplish much of anything, and spies aren't exactly durable. The bots are also surprisingly skilled--spybots will pull off trick stabs like the stairstab and matador, sniperbots have pretty much perfect accuracy, pyrobots will play Ring Around the Sentry if they get close to them, medicbots have perfect uber timing, etc.
The spy's job in MvM is simply very different in MvM than in traditional play. While normally, the spy's primary objective is to take out engineer nests and key targets like snipers and medics, here the spy is much more about sabotage than assassination. You get in the horde, sap them, and run. You sap medic-bots so they can't uber giants, leaving them for your team to mow down. You do still have the task of tracking down and backstabbing snipers, though.
Trying to play a more traditional spy by, for instance, backstabbing the medic healing the Giant Deflector Heavy will just get you killed--the moment you do, your disguise breaks and the Heavy turns on you for killing his medic. You also have to learn the waves and the best times to use your sapper, since unlike in normal play, it has a significant cooldown in MvM. If you sap a group of normal enemies, only to have a Giant Rapid-Fire Soldier hit the ground with 4 Uber Medics on it a few seconds later, your team could very well be in trouble.
Spy may be easier to play in MvM than in traditional play, but I wouldn't say it's any more true than for any other class. It's an entirely different environment with different goals no matter what the class is you're using. The spy's limited ability to kill enemies and vulnerability makes using it well challenging. You're not going to have anything close to the kill count of the Heavy or Soldier--you have to contribute by your few kills being very important and by supporting your team at just the right moments with your sapper.
