More on point two: know when stable enough is stable enough. You don’t need to be cargo strapped onto the machine/bench in 3 different places. Just use a setup that doesn’t leave you focusing too much on having to stabilize yourself

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More on point one: important things are resistance profile of the movement and the biomechanics of it. For example, to emphasize the triceps in a press, an ascending resistance profile is optimal, as most of the effort in the top end of a press is in elbow extension.

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Also generally, the less muscles involved in the movement, the better the stimulus will be for the target muscle. For example, a machine chest fly is only largely utilizing the chest (some isometrics in the forearm and elbow flexors technically but not a huge deal) while a machine chest press is going to heavily involve the triceps and front delts, which can take away some stimulus from the chest. Both will still be great options for the chest, but still something to keep in mind.

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