Despite I have less than 5% rejects of my submission, like with any work, there is always human factor and some reviewers do they job great, but someone…
To be honest, I would say that requirements for items quality, I mean generaly mixing/EQ and mastering are not so strict. Yes, a track could be rejected with such a reason, but then the quality of stuff described above might be really poor.
What is actualy strict - mainly arrangement and track structure. You can make +30 seconds intro (like with some of my tracks which are actually accepted 
), and this is already a reason to reject a track. Or - you can go crazy and make epic solo guitar at the end of the track - and again same reason to reject a track. This little big things is up on the reviewer end, and actually the final decision depends from reviewer qualification and subjective things might have a place, unfortunatly.
Anyway, reviwing process for sure not an russian roulette, you just have to work with your tracks at very least at ‘OK’ level, I mean nice EQ/mixing and mastering as well as following general stock track structure and everything will be fine.