Powershell 3.0+

$wi = New-Object -Com WindowsInstaller.Installer
$wi.SummaryInformation("C:\PathTo\my.msi").Property(6)

Powershell 2.0

$WI = New-Object -ComObject WindowsInstaller.Installer
$SI = $WI.GetType().InvokeMember("SummaryInformation", "GetProperty", $null, $WI, @("C:\PathTo\my.msi"))
$SI.GetType().InvokeMember("Property", "GetProperty", $null, $SI, @(6))

6 = PID_COMMENTS
https://learn.microsoft.com/en-us/windows/win32/msi/summaryinfo-summaryinfo