Well, I can tell you why spy/sniper/pyro sucks:

Well, wait. I actually don't think they suck. In fact, I prefer them. But there is a reason:

1) Sniper is actually better at Demo in fulfilling the role of medic-killer and crowd control...but i takes significantly more skill. If you can't make consistant headshots, you might as well not play, whereas anyone can blow up stickybombs and get at least some damage. Plus, sniper is least effective against tanks whereas a demo is most effective, so there's a pretty huge gap. (Yes, Bushwacka + Buff Banner is awesome, but you're sinking a ton of upgrades into a very narrow utility).

2) Pyro--especially scorch shot Pyro--can be very effective. The trick is, however, that most people W+M1 it, and die frequently. You can't play Pyro even close to how you play it in multiplayer, and the strategy differs based on the map.

3) Spy is also awesome, but the big problem with that class is that the first few waves they are fragile and not as effective as other classes. Once you get a few waves worth of upgrades, it's pretty good.

What these three classes have in common is that they aren't played anywhere near how they are played in standard TF2, and their roles are less defined. MvM only works when everyone is fulfilling their role. A scout that doesn't collect money or a demo that doesn't kill medics is throwing the entire team off. Since spy/sniper/pyro aren't as clear, players tend to play to their own style instead of what the team needs.

Sadly, in my experience, low-tour players who play sniper, spy, or pyro universally don't know how to play those classes effectively. That's not saying those classes are bad in MvM, but that they are difficult to play unless you know exactly what you are doing. Like, as in close to 100% of the low-tour people who pick one of these classes fucks it up.

So no offense, but if you were a 1-tour player insisting to play one of these classes and combating it, I would probably just leave. For every one person like you who might be good, there's a 100 others that it's just going to be a waste of time. That's not be being a dick, that's me having seen this exact scenario play out hundreds of times with the exact same conclusion.