When the seats are available, staff travel is a brilliant perk. Particularly if you are eligible to travel in Business or higher. But to rely on it when travelling on a tight schedule is an invitation to invoke Murphy's Law. With few exceptions, staff travel is "stand by" travel and when things go wrong, the cost saving can quickly disappear. Load predictions can be unreliable as any number of last minute factors may change the availability for better or worse. Priority and years of service in the company certainly count, but when the flight closes and all the seats are full, you don't travel . And remember too that sometimes weight restrictions (particularly ex US) may mean staff are left behind even though seats are empty.
Depending on their contract, some employees can buy upgradeable tickets earlier than the normal 7 years. If your travel category entitles you to buy a business class seat on international sectors, always do so. If nothing else, it increases the number of seats potentially available to you. If you end up travelling in economy, you are entitled to a refund for the staff travel fare difference.
But, as recommended before, an advantage of staff travel is being able to purchase one way travel with no cost penalty. Combining a one way outbound staff ticket with a FF return certainly ensures you will be back on schedule. And in the worst case scenario of not travelling at all, most of the cost is refundable.
I made the most of staff travel for over 30 years and was for the most part very lucky. But there were exceptions. Now I am retired and at the back of the priority queue, I am very happy travelling on commercial tickets. It's a treat to know I will definitely travel and in the seat of my choice. I don't miss the nail-biting wait at the stand by desk and, if it's a good day, that last minute dash to the aircraft!
Best of luck making your choice.
This is probably the best articulated post in this thread. The weight issue is something that is very interesting though and can be deceptive when reading into staff travel availabilities. It's something to note on any flights to the US (in particular DFW) as they will often leave seats empty. I had an experience on staff travel where with 30 seats left (and it wasn't even going to the US), no one was let on.