GaN RF Switch

Disadvantages of GaN
Manufacturing complexity and cost
Material defects and reliability concerns
Integration into existing semiconductor processes
Limited availability of GaN substrates

One downside of GaN is there is no 300 mm GaN wafers. GaN wafers are usually comes in 150–200 mm sizes - it makes them unattractive for ULSI.
Semiconductor industry is driven by economics. This is why all major companies are producing their products on 300 mm wafers because this way more dice (chips) produce on the same wafer.
GaN is superior to Si in mobility allowing faster transistors but inferior in mechanical and thermal properties.

check these links for more info

What You Need to Know About GaN and SiC FETs

New FETs on the block. What you need to know about high electron mobility transistors (HEMT) silicon carbide (SiC) and gallium nitride (GaN).

resources.altium.com

hanif said:

One downside of GaN is there is no 300 mm GaN wafers. GaN wafers are usually comes in 150–200 mm sizes - it makes them unattractive for ULSI.

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With only a N-type, ULSI is not in the picture. GaN
vendors manage to integrate the gate driver. Memory
and low power logic, you cannot have in today's GaN
technology space. Analog, got plenty of figuring out
to do, probably taking cues from what's been doable
in GaAs for support circuitry (pain and accomplishment)
and from earlier NPN-only technologies' design styles
and addressable problems vs not-.

Now one of my notional technology developments
is a path to co-processed CMOS and dGaN (or eGaN)
on a single insulating substrate. If you're up for fun
and have a couple of million to throw, at a do-all
PMIC, VLSI digital, LSI mixed signal and lower cell
bands RF, for extremes of temperature (77K to 300C)
and radiation (do not ask, in an "international forum")
be sure and get in touch.