What are advantages / disadvantages of horizontal and vertical microprograms?
This questions seems dated, about the way computers and instruction sets were designed in the 70's.

AFAIK, micro-ops in x86 CPUs have about 100 bits, and are decoded separately in each operation unit (ALU, LSU, FPU...) There is no gigantic flat microcode controlling every multiplexer.

There used to be several layers of microcode and ROM decoding tables instructions are translated into "vertical" micro-ops which are then decoded into "horizontal" signals (IIRC, the MC68000 was like that).

Pure horizontal microcode is impractical in modern chips. Long wires have parasitic capacitance and are "slower" than transistors. Using a large and wide microcode ROM outputting hundreds or thousands of signals to individual adders, multiplexers and registers would make a large and slow chip.

Additionally, nowadays processors are pipelined : A wide microcode would need hundreds of flipflops to propagate instruction decoding along the pipeline.