I don’t know how it is now. During the Vietnam era, I always thought the Army was the easiest place to make rank, we had two year service draftees who were E-6’s. The Marines being in second place. You could do four years in the AF and still be an E-4, in fact that was typical. If I had extended for 12 months after my two, I could have made E-5, I had way more than enough points and I am pretty sure my CO would have recommended me.
To brag: I was meritoriously advanced from E-4 to E-5 on a ship of 600 without all the studying and tests. I even have a letter from the Captain congratulating me. And no, I wasn't a suck up to my division CPO or Department Head who both put me in for the advancement. I just did my job on the flight deck.
I was surprised at a ship's morning muster when the Captain called me forward after assigning ribbons/medals on others. I kept thinking what did I do wrong? Am I going to Captain's Mast? Instead, he handed me my E-5 crow and shook my hand and thanked me. Best Captain ever. The ship was always in tiptop shape and battle ready. It won many Battle Efficiency Awards. You can see the big yellow E on those ships.
He also signed a personal message on a photo of the ship saying, "Your Meritorious Advancement and now your Re-enlistment. I hope I'm around when you make CPO". That photo hangs with my other Navy stuff to this day. Still wish I had stayed to make CPO (did make E-6 before final discharge), but my rating was closing up and had a lucrative offer from a civilian company. Ok, end brag. Sorry to bore you.
The above said, my greatest respect goes to Marine and Army grunts who are more in harms way than my Cold War encounters with Soviet subs.