Curious that there seem to be a lot of people on this forum focusing almost exclusively on non AMEX providers for card sign on bonuses.
As a rule, I find the AMEX application and approval process so much easier than other providers. This plus the fact that they seem to credit account holders with multiple (and generous) initial sign up bonuses through the referral scheme, means I focus most of my time on AMEX applications these days. Usually there is no request for sending additional documentation, and the card simply arrives in the post (as opposed to banks, where you need to go into the branch with ID to pick cards up). It's easy to cancel the cards, and you can even get a generous pro-rata refund - all in all, AMEX are a card churner's dream
The absolute worst provider I've dealt with has to be Macquarie (the only one I have been rejected for). Even though I uploaded payslips and notice of assessment upon their request, they demanded to phone my employer to seek proof of my earnings, and despite me trying to coordinate this with their incompetent call centre for the best part of 3 weeks, I later received a rejection letter stating that they couldn't progress my application further due to not being able to confirm my income.