Again....I am a professional grower and as such have a vested interest in this discussion but that does not mean I am necessarily biased.
There is a veritable cornucopia of tropical lilies available from licensed growers compared to the mere handful offered on ebay ....and without exception, I do not know of a single licensed, professional grower that even considers selling tubers. the whole point of being a "grower" is to grow things to a marketable size.
Given these are tropical plants, the season is indeed limited in much, if not most of the US, and a established potted plant will not miss a beat blooming or be of concern should a late season front sweep through and drop the temps for a day or two. By contrast, a tuber is a crap shoot as to if and when it will bloom and a cold night or two is much more likely to send it back to dormancy.
Do full sized, established plants cost more?...sure they do. A grower has done the work for you and all you need do is enjoy them. A tuber is fine if you want to be the grower, but at least a third of the blooming season will be lost to you, at really a minimum of savings.
About all tropicals lilies by necessity come from a climate "different" from most of the US. But I know of none grown in a 'greenhouse', unless that is an aquarium in a window<g> And would love to see some supporting data to the claim there is a "high probability of plant shock" of a well grown plant, when compared to the shock experienced by a fragile sprout from a tuber.
Truth is not what you want it to be; it is what it is. And you must bend to its power or live a lie.”― Miyamoto Musashi
"Knowledge speaks, but wisdom listens." ~ Jimi Hendrix
“The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge.”― Stephen Hawking
Craig