Well from a strictly lore perspective it makes sense that Caliban's Adaptation would only work at 50% efficiency creating a Sentient Warframe Hybrid would in theory have its drawbacks you are in theory merging two completely different and incompatible entities thus the their Adaptive systems would work at a reduced rate due to IDK instability or perhaps just incompatibility.
But I suppose when Caliban Prime rolls around in MAYBE 3-5 years we might finally see him be "Viable", I mean really when I play him the only ability I use is is Lethal Progeny infinite overshields plus if you have all the Umbrals installed his Sentients spawn in at above level 50.
I've also frequently wondered if his passive effect applies to defense targets (So long as he remains in close proximity) but I can find no information proving or disproving my theory anywhere, but it certainly does feel that way for example when camping on a hijack target while his sentients repair his shields and keep the crowd busy.
I do however have to agree with you Chroma hasn't felt "Powerful" for me in at least 4 years, his spectral scream I think would be better if it dealt the 4 primary elements simaltaniously as for his sentry ability I think it would be better if it IDK maybe had the ability to fly around and not be a sitting duck, anyhow...I remember I abandoned him as a "Main" when Inaros Prime Launched I vastly prefer his tool kit of life stealing best choice I ever made.
A bit more wordy then I anticipated but it happens.