No matter how hard you try, there will be lots of stalling, postponing and no replies from authors
LoR authors are not the best people to write such things!
Your letter might be a copy paste material from other letters
Your letter might be written by someone who you haven’t dealt with such as secretary
You would experience the worst feeling of tracking, chasing, pursuing and pestering someone for such a small favor.
They might reply, ask you to write lots of stuff, tell you to send them the codes to upload the letter and BAM BAM; they won’t upload the letter and never answer you back!
Most stressful of all the actions in the match as you have no control over it.
But look at the bright side, there is always a plan B; the emergency plan! All programs request 3-4 letters of recommendation, prepare yourself for 5-6 letters then! If anyone fails you, or the letter they have submitted doesn’t suit your aim; then you would have an extra one to fill the space. But don’t upload less than 3 letters! If you upload 2 letters; that gives an impression that no one wanted to write you a letter.