Fixed instruction encoding. Minimal instruction set with 32 GPRs, R0 is always 0. Absolute addressing possible using R0 as the base. Instruction mismatch is likely, in which special instructions do more work than is required for the frequent case. There is no such thing as a typical program.
Programs can vary significantly in how they use an instruction set.
Many times it is meaningless to average frequency criteria over several programs. Figure 2.31 shows a large variation among the SPEC92 benchmarks. An architecture can have flaws and still be successful. Used segmentation, everyone else used paging. Used extended accumulators, others used GPRs.