I just read this opinion over on CoinTalk (I'm not a member over there) :
I just can't get behind any Roman coins from the Constantine era onward, I find them just boring, no variation in the busts, with same reverse designs, the only aspect that makes the late Roman coins interesting (at least to me) are the different mints.
I think this is probably a fairly widespread opinion, with Constantine-era coins regarded as the realm of beginners, but how wrong can you be on all fronts !
For Constantine we have over 100 reverse types (never mind sub-types and varieties) and I wouldn't care to guess how many bust types. From what I've seen the bust variety on earlier imperial types doesn't even come remotely close in terms of variety. Even London 311-312 alone, or Crispus's Trier, would seem to put most other emperors to shame !
I guess from the outside looking in, it may seem that Constantine only issued soli invicto, votives, campgates & gloria exercitvs, and I guess anyone only looking at these 4/100+ uber-common types is consequently also missing the bulk of the bust varieties.