SOI is one way of isolating devices. I prefer it. I've worked in,
and on, SOI technologies with CMOS, bipolar, BiCMOS device
sets. Dielectric isolation, bonded wafer, SIMOX and silicon-on-
sapphire. You can do anything you want if you are willing to eat
the wafer cost.
I wouldn't try bipolars on SIMOX or SOS, the defect density is
too high and you get too much device-device lifetime variation
(lifetime being key to hFE) and excess noise. But I've done a
whole lot of linear ICs on DI and bonded wafer, in a prior life.
Digital too, back when bipolar logic was still faster than CMOS.
Given a choice I stay away from JI technology. Cheap is its
only real upside, and I don't do cheap.