Having sat on the other side of the desk, until I retired:
1. To an extent you get what you pay for. No different to your car/bike/hifi/camera
2. Some lens designs will work better for certain tasks. It's your Optician's job to advise you on that. Lenses can be grouped as "hard" or "soft designs. This refers to how fast the image quality falls off to the side of the reading area. The power build up can be quick or slow (corridor length).
3. Some folk are much more, or less, tolerant to the "swim" effect. A good quality lens nowadays will give little of this peripheral distortion. Some of the early types were awful.
I was in the fortunate position of being able to try many different designs. The manufacturers are keen for one to recomment their product and used to provide complimentary pairs for evaluation. I hated some and loved others. That gave me reasons to recommend certain lenses for certain tasks.
The analogy I used, to my patients, is that varifocals are like shoes. If I give you half a dozen pairs of different shoes, in your size, they probably won't all be comfortable. They WILL be your size though. Part of the professional time you pay for is to advise what is best for your needs and your lifestyle.
Not really any different from buying a bike. You need good advice to get the best result
EDIT - The fit matters A LOT too. The curve of the frame, the angle towards your cheeks and how close they sit to your eyes can make an enormous difference to the lens performance