The way I've heard it from a couple of very reputable smiths that frequent this site is that due to the larger case head of the Lapua, more force is exerted rearwards by the larger case head against the bolt lugs. 65K psi pressing back against the larger casehead of the Lapua gives said pressure more surface area to push back against, and thus more overall rearward force, than the standard magnum sized casehead of the RUMs.
It is the strength of the bolt lugs that is in question, generally, to my limited understanding.
Many smiths will not build a 338 Lapua for you on a stainless 700 action for this reason. They think the stainless bolt lugs do not posess enought of a shear strength safety factor. They may not want to be, in any way, responsible for a bolt sticking out of your head.
Some will build on a stainless 700. Some folks on this site have 338 Lapuas on stainless 700 with stainless bolt, I believe, and report no problems.
I think that most smiths will build a 338 Lap on the standard 700 chrome moly receiver as it is stronger than the stainless.
A compromise that I talked to one smith about was to use a stronger aftermarket (PTG, for instance) chrome moly bolt in a stainless 700. The aftermarket bolt from PTG being stronger than the stock stainless 700 bolt.
I agonized over all this info for some time and finally ended up going for a BAT HR PIC action for the Lap Imp 338 AX (Allen Xpress). Integral recoil lug and 20 MOA base in a fairly lightweight 31 oz. package.
I'm no smith, but have been where you are and looked at this from every angle I could conceive of and finally bit the bullet and spent the cash.
Good luck--Jon