One positive note: Telperion Farms uses Root(grow)bags, allows big leader to grow trunk and trims low branches some to keep main tree "developable" as Bonsai tree
. "May" also dig bag and prune roots but guessing here.
The specific methods used by top growers such as Telperion go a long ways to address the problems with in ground development. Bonsai enthusiasts are fortunate that growers continue to advance the knowledge needed to produce fine Bonsai. The more experience one has with various methods the more clearly one understands the trade offs.
Initially in the early growth years the work required to produce movement, Nebari and lower branching suitable for fine bonsai is considerable and frequent. This slows growth at various intervals and also adds to the cost of development in both labour and growing time before ready for sale. Trade offs!
For purposes of this discussion the question was are there disadvantages to growing in the ground? The answer is yes, but there are also advantages initially. My experience is that the time gained initially is spent reverse engineering problems that can develop with rapid growth. Or as is often in ground growing uninterrupted growth. The absence of steps taken to control nebari and or lower branching development. The better the grower and their process the fewer the growth issues. The more extensive their process the higher the cost.
Also, it should be noted that the ability to select from hundreds of trees developed in ground sure beats growing one or two in ground at home. in most species correcting root problems is possible and lack of lower branching can be addressed with grafting. One just needs to be realistic about the time and skill involved if those issues exist. I work with stock i have purchased from Telperion and fully appreciate the extra steps that they take in their process. Some of the work they have pioneered played a direct role in the methods I have chosen for my own nursery. I consider them to be one of the best at what they do.
PS: Not to mention the problems of working with the lower branches of trees while laying on the ground!