Problem with the 60 was it lacked Split I/D (as did the 40's).    We kind of relied on that for the kernels towards the end of the PDP-11 days,

We struggled with the lack of I/D on the 11/34 and 11/23 at BRL but finally gave up when TCP came along.   We just didn't have enough segments to handle all the overlaying needed to do.    I recycled all the non split-I/D machines into BRL GATEWAYS.

Of course, there was the famous (or imfamous) MARK instruction.   This thing was sort of a kludge, you actually pushed the instruction on the stack and then did the RTS into the stack to execute the MARK to pop the stack and jump back to the caller.      I know of no compiler (either DEC-written or UNIX) that used the silly thing.    It obviously wouldn't work in split I/D mode anyhow.   Years later while sitting in some DEC product announcement presentation, they announced the new T-11 chip (the single chip PDP-11) and the speaker said that it supported the entire instruction set with the exception of MARK.    Me and one other PDP-11 trivia guy are going "What?  No mark instruction?" in the back of the room.