If statutory declarations started getting rejected by DIBP, as a matter of principle, then half the members here can forget their grant.
As per my understanding, DIBP takes a dim view of those SDs which do not have multiple evidences to back it up like payslips, Service certificate, bank statements, tax statements.
If you can provide all 4 evidences for the period you have given the SD, then I don't think you need to worry.
But you should also be prepared to get a call from DIBP for verification of your RNR and also to the HR department of the company for which you have provided SD
Cheers
That is basically what I've meant by using them only as "last resort".

Even if getting a proper reference letter written seems complicated in the beginning, it definitely makes things much easier (and faster) in the end.
According to a guy I know who works as migration agent, the whole verification process usually takes much longer once a SD comes into play as the DIBP runs more background checks and your chances of maybe even getting a direct grant drop to around zero. Also you have to inform your references in the SD that they likely are going to receive a call by either the ACS, the DIBP, the Australian Embassy and/or some other government institution, so you'd need to be in touch with your former employer anyway. And then who knows who is going to pick up the phone and what he/she is going to tell the person calling?
Of course things might work out - but with an SD you're introducing a whole new layer of complexity in the application process, that you can't control. There are quite a few threads from people who had their visas rejected due to inconsistent answers the DIBP has received when verifying SDs. Since there is a lot of money involved, my recommendation would be to play it safe (whenever possible).