A good question and more than a few go there and even build batteries from old laptops. Yes the things you mentioned but to me, the biggest problem is, lower quality cells means capacity difference among cells which translates to a battery going out of balance faster, in particular if you don't top balance the battery with a BMS...how most work...in an effort to prolong cell life. Charging cells to maximum voltage may help balance a less than uniform capacity battery but in doing so shortens the life of the battery.
Another way of looking at this philosophically which we learn in other life endeavor's is, there is no free lunch. Some would consider B grade cells a false economy. What is the cost you may wonder for lower cost cells even failing a bit faster? Your time. The hassle of dismantling the battery and fixing it when it falls out balance or building a replacement battery sooner and hassle of a BMS shutting off during a ride due to an out of balance parallel group within the battery.
Perhaps the most critical factor and most unknowable factor if buying a battery and not building it is....how good are the cells and how equivalent are they in capacity? When buying a battery there is a leap of faith and for some purists this is a bridge too far and why a small subset of the ebike community build their owner batteries and even fix them when they fail or start to wear out due to not having the best cells to begin with.
My thoughts.
In life, there is diminishing return with all things. If building a fresh battery some will capacity test every battery and then isolate them for decay for a period of a month or two. Others will pull them out of the box, randomly pick 10, load test them and if close enough for horseshoes build the battery. Labor and time has a cost and many don't believe its even worth it to build or even repair and aging battery. Honestly, I am slightly conflicted about it as well, but personally I would never build a battery with B level cells for reasons mentioned. Not just performance, but staying on the road and not always working on batteries that are more prone to failure due to lower quality cells.