I only use COMC to buy base cards for sets I am working on.
The first issue for me is that new inventory does not get added often (at least for the sets I'm working on). If the common I am looking for is not available in June, its still probably not going to be available in November.
The second issue is pricing. For some cards I want/need, the prices are ridiculously out of line with reality, probably because the sellers do not bother to adjust their prices to the reality of the market.
The third issue is that for the most part, COMC rarely has inserts from the sets I am working on. In all honesty, I don't think I've ever spent more than $5 on a single card on COMC because of this. Anything high-value is going to show up on EBay, not COMC.