Standard protocol (Probably IATA standard) is the operating carrier of the first flight checks you in for all segments on the itin (assuming there is an agreement for such). *A carriers should all be able to - though some others can't, even if booked through UA.com (for example, I was on a CX-UA connection, and they checked bag through, but couldn't check me into onward UA segments.
Never had a problem with through check-in and getting BPs when checking in with NH or LH and continuing onto UA. in these cases, should always get checked in without a problem, even if BPs can't be printed. You can always pick those up at the transfer desk, or as already mentioned, once landing on the last OAL flight, and only UA segments are left, just open the UA app once you have a cell/wifi connection and the BPs should appear, including with precheck if applicable.