Just how good is their quality? Would the assembly make a meaningful difference? I.e. would it be worth having it put together by a luthier?
My experience is that they've a very consistent production... Yes, the assembly will make some difference, as it'll need a small fret leveling, specially if you use low action (sometimes it's not necessary even for those with low action, but they don't make any fret leveling)...
Their options are wide, but it's hard to get something that is not listeded as a option, because they're not luthiers, they're a guitar factory that produces in line style... if your plan is swamp ash body with maple neck/fingerboard will be absurdly easy... If you want the same things as Captain Desslock then you'd better just go to luthier...