Get it in writing and check the fine print. A recruiter might lie but an enlistment contract is exactly that: a legally enforceable agreement. If the contract guarantees a particular MOS and the recruit doesn't get it for whatever reason then they have the option to take a discharge instead of being reclassed.

Even without a written guarantee, in volunteer forces the senior officers will generally get rid of new recruits who decide they don't want to be there instead of forcing them into a different job. Especially for the combat jobs where even training can be deadly. Better to give them discharge papers and GTFO rather than wasting money training someone who's going to be a poor performer with constant morale and discipline problems.

(Of course if there's ever another world war and conscription is reinstated then the rules will go out the window. But that's not what we're discussing here.)