First off: vested interest - I work for a MMC company (SIP Energy Ltd).
The single biggest issue with any off-site made home, is not the off-site part, it's the on-site part.
A key question for us, when we started, and for the BRE who audited us, is on-site quality. And the single biggest reason we came to for this, is that most TF company's do not use their own crew. They sub-out the job to local erectors, and they are paid, usually, on a per sq ft basis. This, imho, is a mistake. If for no other reason, the subby (who's usually been beat-down in price to get the job btw.......), needs to get in and out of site a.s.a.p., to avoid losing money on the job. This then, is anathema to putting quality first.
So, the question you need to ask is, who are the frame erectors, and who do they work for ? In other words, what is to make sure they put quality first ?
Other things to consider: who's supplying and paying for the crane to put in the system ? I have seen more arguments over this than nearly anything else. I've seen the TF include an 'allowance' in the house kit to cover the cranage, but the erectors then telling people (usually correctly, imho, btw.....), that the TF company didn't allow them enough in the budget for the cranage, and so the row develops over who's paying for it. And with a City Crane running at Eur600/day, it doesn't take long to get out of hand. Want to guess who ends up with that bill, ultimately ?
So, crew that works directly for the factory, and includes ALL cranage - irrespective of time allowed, are, imho, two big ++ for any company.