The suitability of live-view and mirrorless camera options for use with long telephoto lenses comes down to ergonomics, practice, and context.

The biggest factor is likely how you're viewing, sighting, and stabilizing the camera, vs what you are photographing.

Viewing a non-tilting rear mount screen is probably the worst option in nearly any context.

• You give up stabilization to hold it out from your body enough to view the screen.
• You leave the camera 'floating' in front of you more, reducing your bio-mechanical feedback with where the glass is actually pointing, and reduce your ability to reliably sight 'over the barrel' to approximate your aim before locking in on target.
• Increased risk of issues such as screen glare making it difficult to see.

An eye level viewfinder is typically the most favourable option for the majority of use cases.

• You gain stability by bringing the camera close into the body.
• You gain bio-mechanical feedback by typically using the same 'contact points' with your body, as your hand and eye consistently align while you swing the camera.

*However, there are scenarios where one may benefit from using a rear screen, especially when tilt options come into play. [These may also apply to electronic viewfinders that can be viewed independently of the camera, such as 'google mount' options.]

• Positioning the camera lower than a traditional viewfinder allows
• Working in hazardous situations where gluing your eye to the back of the camera may make it more difficult to notice a danger in time to react.

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All that said, getting a long lens to hold on target is a skill, and it demands practice.

But if photographers can learn to track moving subjects with a waist level finder on an old medium format camera [where the view is flipped left to right], then odds are good that you can learn to adapt to a given setup.