I'd say combi-flamers aren't even that great. Plasma is better, or full melta
imo. You already get rounds which ignore cover, but no
str:7
ap:2 rounds for dealing with terminators or other such tough units. Sternguard are expensive too, flaming a few orks or guard won't help them make their points back. However taking out an entire
TEQ squad and a tank, or taking out 2 tanks, or even smooshing a tervigon is definitely worth it.
Best sternguard I've seen is drop pod, 10 melta, combat squad, waste two of my tanks on turn one. It's annoying as hell, but it's a great tactic.