My worst experiences with beta readers is those who up and abandon you in mid-rewrites and you are waiting and waiting for the e-mail in your inbox with your next chapter ready for your eyes to go over them and do the corrections needed.
Only to find that the beta reader has not sent you anything. You figure just wait a little longer than the week you have been waiting. Finally, three weeks have passed and you e-mail them to see what is going on with the edits to your story and when will you see the chapter come back into your inbox. After a month of not hearing from them at all, they finally respond to you stating that they got involved in with something else and you should get the next chapter in a couple of days. You are okay with that and patiently wait or continue writing more chapters to your story till the second day comes and you check your inbox to find nothing from your beta reader at all.
The two day promise didn't happen and they didn't send you the chapter when stated. You e-mail them asking for the chapter even if it was not finished being edited yet as you're firing them as your beta reader because you need a beta reader who is more responsible as in doing the editing and sending it back to you within a few days.
I had to drop several of my beta readers that I have had over the years, all because they don’t send me, my work back when they promised to return them. I would have understood if they were delayed in working on my story, only if they told me so. I would love to have a beta reader stick with me longer than nine chapters but that means finding one who you have to pay to edit the story and you don't have much money to afford paying a beta reader. Finding a beta reader for either fan fiction or original stories is like pulling teeth.
It might be for the best interest in any writer to start to learn how to beta read your own stories and bypass the beta readers themselves since most of them will not finish the job you have asked them to do or spend money on possible scammer beta readers who will do a shitty job and you end up finding more errors than anything.
I may consider doing my own beta reading myself on my stories and not contend with having beta readers at all.
