Was taking a Yak-38 against a powerful fighter any worse than taking a Harrier against a MiG-29 or Su-27?
Short answerYes in this comparison: the later Sea Harrier had beyond-visual-range weapons and good missiles, while the assessment of first-generation Sea Harriers is more cautious.
NotesThe Sea Harrier Mark II had beyond-visual-range capability and effective missiles, giving it a chance against more capable opponents. That equipment is the reason offered for treating a Harrier engagement differently from sending up a Yak-38. The judgment is more cautious for first-generation Sea Harriers against the Soviet fighters named in the question.