I guess there are advantages and disadvantages to each of the wireless protocols. I have a mix.
Most of my lighting uses Lutron Caseta Clear Connect which means my light switches, dimmers and PICO are on their own hub and interface with Hubiutat using Telnet. The devices work well.
I do have a few Hue bulbs which use the Hue version of Zigbee. I only use Hue for lamps with single bulbs that plug into an outlet. Lamps with multiple bulbs are on Lutron plug-in dimmers. For a lamp with a single bulb, using Hue is a less expensive option. So far, I have not used the LED color capabilities.
Most of my sensors are Zigbee HA with varying versions depending upon when I purchased them. They generally work well.
I have started to add some Z-wave devices to my network. I have a Schlage Connect lock that uses Z-wave. I had to add an Aeotec range extender 7 to insure reliable operation of the lock. I had Smartthings multisensors on the garage doors using the x,y,z tilt sensors to monitor door opening. However, since they are no longer available, I have switched over to Ecolink tilt sensors that use Z-wave.
I know there are some advantages to having everything on one protocol. However, sometimes the "best" devices for a specific application use a different protocol. Thus, I have decided to have several systems with enough devices for reliable communication.
Thus, if you are faced with a situation in which you have older Z-wave devices that need replacement, do some research to determine whether one system (Z-wave or Zigbee) can do it all, or whether multiple system will do it better.
Also remember that Z-wave is currently in a transition phase. Some Z-wave Plus devices use the older 500 series chips while some of the newer devices use the 700 series chips. If you stick with Z-wave, do you purchase 500 series devices or wait for more 700 series devices to hit the market?
The latest Hubitat hub uses a 700 series chip, so I purchased Aeotec 7 series range extenders to match. For some devices, it might not matter whether they are 500 or 700 series chips, do the research so your money is invested wisely.