I have had several wonderful Beta experiences. Wonderful ladies that I swapped chapters with who offered advice/critiques for me and were thankful for my help/comments on their work. My writing has improved by working with them.
I had a bad experience I want to share but it was as a Beta. I had a writer that I found through another site that traded with me a chapter at a time. She continually assured me that she would implement the grammatical and other simple changes when she could so I continually had to note the same type of mistakes, hoping against hope that I would reach a chapter where she had stopped making that same error. I know. I should have kicked back the first chapter where I saw the errors continuing but again it was on trade and she was editing my work.
About half the way through, I noticed all the conflict in her story happened to the buddy and not the MC. I explained my point and encouraged her to add more conflict. She thanked me and went to do a re-write. Months later, she then forwarded her MS and asked for my thoughts. Again, I took the time to read the whole thing (250+ pages), only to find that other than some (not all) of the grammatical errors, she implemented only one very minor change and left everything else as is. Now I don't expect a CP to take every suggestion, every writer has their own vision but I felt I had pointed out a bit of a misstep, and that it was something that would improve her story. So, maybe she disagreed but what about all the comments where I couldn't tell who was talking because of a lack of action tags and dialogue tags? These things were the same, too!
My point is that my time is precious, and if I'm doing Beta reading, I'm not writing. If you aren't interested in my perspective, then send me on my way. I can read for someone else. Why would you ask me to continue to read your stuff, not implement any real suggestions, and then expect me to read it again and comment? If you are not interested in a different set of eyes to review your MS, don't keep asking, hoping for a different result. Isn't that the definition of insanity?
The oddest part was she thanked me in her acknowledgments but not as a Beta (apparently she had others) but as someone who helped her along this journey? When she asked me to comment, I tried to beg off and say a generic "I like the changes you made and congrats on finishing" but she came back and asked if I had any further comments. Any suggestions?