A lot of us play engineers very differently, and some of the models have different weaknesses and strengths based on our own lineup more than the opponents.
I'll give it a go though.
Hoist is not that great on paper. He's a slower velocity that just doesn't die without an entire teams worth of influence put into him. He really shines with teamwork and his one character play, true rep.
Whether you want to go all striker by stealing burrow from mother or acrobatic from velocity, or maybe be cagey and dangle him infront of your opponent and throw blast earths, you can't go wrong as long as you plan your game around him and don't overextend him (or do, he just wont accomplish anything after that)
Good into: yes // risky into: poor positioning.
Mother can sprint 7. Then place a marker 4 inches away. From this point, you add the base size and choose your options. Grab a free ball base size +2 inches away from the marker for a ~14 threat on a free ball. Burrow to the marker for a 11+30mm base + 40mm base +4 inch kick threat on goal (about 17.75 inches). Or just leave markers on the field and have hoist teleport like houdini. Really as long as you know your ranges and threats from models and goals, there isn't a wrong play besides being out of range.
Good into: playing the game // risky into: seeing mainspring on the field instead
The only bad part of mainspring is that he's not mother. This mascot handles the ball like no other between a 6/8 move, free pass with 3 dice as well as free long bomb, he enables snapshots for days. As long as you have the ball. Bad part is the opponent also wants the ball so he only ever sees it once, maybe twice a game and its not super easy to turn that into goals when he's supposed to be efficient meaning no inf given to make any shots forcing you to have a snapshot turret ready to go.
Also is not mother.
Good into: receiving // Risky into: not being mother
Ratchet is the mechanic your mechanic wishes he could be. No robot wants to enter the pitch without a checkup from this guy. His biggest weakness is the other team in all scenarios. 3/1 1 inch melee with no double reposition outside the unattainable 5th column with 5 tac? he screams please punch me I'm starting to like it. Upside is he is the most efficient player on your team. Free condition removal for mechanica, tooled up, heroic for a free sprint/charge on mechanica models, heck even longbomb for the surprise snapshot goals from way downtown where he belongs. Giving him 1 inf for tooled up or 4 for blast earths with Ballista (or pin vice, doesn't matter) isn't even that bad an idea, especially on turn 1. Just don't let the opponent touch him without your consent and he'll fix your team right up.
Good into: Playing the game // Risky into: poor positioning.
Compounds greatest strength is being really hard to touch for your opponent. A lot of good ranged control effects require direct targetting, meaning gluttunous mass protects the counter charge aura from being turned off without a model physically blocking him. This is great for a guild like engineers who are capable of sitting back and abusing the countercharge aura to force the opponent to commit poorly while throwing ranged character plays at will. He is even great when the brawl starts as any model trying to escape him risks a knockdown unless they buy two attacks to knockdown/push/dodge away. He doesn die easily to a concentrated effort, so don't overextend him into a setup beater without a safety net. Memory is his worst foe due to dodging, but not unplayable into.
Good into: Playing around him // Risky into: poor positioning/brainpan and memory