I was still very doubtful about the chances for success.

我仍然不能肯定成功的可能性有多大。

《柯林斯英汉双解大词典》

The effect of whether "exact", it is very doubtful.

这些疗效是否“确切”，是非常值得怀疑的。

youdao

Whether we could thrive on such food is very doubtful.

我们是否能够培育出这样的食物，也非常令人质疑。

youdao

The 95 B-52's are all over thirty years old, and their ability to penetrate modern air defenses is very doubtful.

CENTERFORSECURITYPOLICY: Letter to the President

So I don't think anything beyond that, even higher up that people are suggesting that we might find out, I'm very doubtful that we will find that out.

NPR: British Prosecutors to Receive Russian Spy Case

The ruthless attention to detail and complicated build means it is very doubtful that it will make them any money - and yet I'm not sure they care all that much.

BBC: Trick of the light: A California road trip