Here are the issues you could encounter :
Issue 1 : The reminder is not sent for pending Posted Approval Workflows, when they are triggered manually with the “One Step Approval” button.
It is fixed in SP4
If you don’t plan to install SP4 now, a CI exists to fix this issue. Download it : vim_7.6_sps3_vimi-29524_ci-129
To be completed with other issues.