Guaranteeing uber to airport

No unfortunately you can't guarantee.
If you normally struggle in your area then it could be risky.
From amwaj it can take up to 30 mins to secure a driver so I always allow that much spare time.

Careem is available in Bahrain now, they do allow pre-booking. I've not used them here but in saudi they were really good.

You can sign up here Careem Bahrain  - - https://www.careem.com/signup/KM68MQC7IM

thanks I'll try that

You can pay by cash too which is handy, though I prefer the convenience of card.

You can book a taxi using http://www.speedymotors.com/ taxi service, I used them in the past and they were reliable (just call them to confirm the booking as waiting for a reply via email/sms is taking a lot of time). 

If you are using uber you will find that the same cars/drivers are also replying for uber calls (I was suprised to see that) but you cannot book for future with uber.

Note that using speedymotors you will need to pay in cash (unless they changed that now)

But Speedy is more expensive.

Usually the Uber driver is a regular Wasel taxi car but the rates are lower when arranged through Uber.

I tried to use Careem last night and you can't actually call a cab! It's a booking only service it seems, for now anyway. So if you do want to use them then sign up and book it at least a day ahead.

I find picking up the phone and calling a taxi company is always the best option and they have never let me down. I don't trust apps especially when you have to register credit card details and you are on a tight schedule. Speedy and Arabian motors are both excellent and reasonably priced.