That instead of getting a geometric amplification you get a linear one, which much less product in the end.

UP-PCR (Universally Primed) uses only one primer dimer (sequence of 10 nucleotides). Also expect many bands (amplification). Scoring the bands for genetic polymorphism is tediuos for large number of samples. One more thing, the PCR reaction may take longer (3-4 hours) to complete the reactions.

Agree

Arithmetic accrual (10, 20, 30, 40 ,50...) of product per cycle, instead of log base 2 geometric accrual (10, 20, 40, 80, 160, 320...)

I don't know reason of that but I mistakenly ordered both forward primer instead of one of each but I didn't know I did PCR got two bands 300bp and 100 but originally product size was 450.