Controversial Caveats:
Shorten life of shingles (time=heat physics)
Incomplete blockage of vents, ridge & soffit
Open cell can absorb & conceal roof leaks until too late discovered, $$$-Closed cell much less likely
Gas appliances need special venting if in such a sealed attic
Off-gassing invasion into house - at least for awhile
More money & SF covered for same house insulation R-values via min.wool, fiberglass
Any penetrations invite humidity problems
The long term consequences in Texas (~10 years) may slowly exhibit themselves; CO2, et al.
Foam = ideal for many buldings & applications where residential R-economy ones cannot apply
Since this is not the EZ SOP & popular opinion in residential build circles, I expect this to draw a lot of heated fire.
Oh, well.
3000LivSF 8:12 roof
Homewyse.com upper end cost.proj
R11 foam rafters $27,200
R19 blown floor $19,200 +72%R @ 70% cost
Ten words or less ... a goal unattainable