What happens to shower temperature when another tap opens?

If someone opens a hot or cold tap elsewhere, what can a thermostatic shower valve realistically hold steady? I understand it’s meant to regulate temperature, but I wouldn’t assume that means it keeps the shower’s flow or pressure unchanged too. How much does the result depend on the house’s supply conditions and the valve’s compatibility with the plumbing?

I’m also weighing the added temperature control against long-term upkeep. Are replacement cartridges generally available years later, and does the valve need enough access for service without tearing into finished tile? Which of those details would matter most when choosing one?