It seems ok to me. But I'd use

I'm doubtful about his experiments.

And you could also use other words like questionable or unreliable

His experiments are questionable / unreliable.

It seems ok to me. But I'd use

I'm doubtful about his experiments.

And you could also use other words like questionable or unreliable:

His experiments are questionable / unreliable.

It's seems ok to me. But I'd use

I'm doubtful about his experiments

And you could also use other words like questionable or unreliable

His experiments are questionable / unreliable

It seems ok to me. But I'd use

I'm doubtful about his experiments.

And you could also use other words like questionable or unreliable

His experiments are questionable / unreliable.