Comparing dates with SharePoint PnP PowerShell
I think it's okay, but we can do better. Let's see what Get-Date returns. Run this:

&gt; get-date | get-member


It shows that Get-Date returns a DateTime object, and it also lists all the properties that has. I notice that DateTime has a Date property. Looking on the Web, it says that this is the date part of the DateTime but with a zero time component. So let's use that instead of converting to "yyy-MM-dd":

if((Get-date).Date -eq $ListItem["Aviseringsdatum"].Date){
Write-Output "True"
}


Since this is a code review, let's improve the formatting, and indent and add spaces:

if ((Get-Date).Date -eq $ListItem["Aviseringsdatum"].Date) {
    Write-Output "True"
}


I think that's as far as you need to go. If it were my code, however, I would probably go a bit further. 

I would use [DateTime]::Today instead (Get-Date).Date because I think it is clearer. 

I would also introduce local variables. I think if you are ever using a "today" or "now" value in your code, then it's better to capture them in a variable for the sake of consistency so that all parts of the script agree on what "today" or "now" is. What if for instance your script ran over midnight? Then the (Get-Date).Date value would suddenly change. A local variable might be overkill in this case, but it's a good practice in general to avoid some subtle bugs.

I also like making local variables to capture bigger expressions because they make for self-documenting code. It might be overkill again in this case, but I will add a $notificationDate variable.

$today = [DateTime]::Today
$notificationDate = $ListItem["Aviseringsdatum"].Date
if ($notificationDate -eq $today) {
    Write-Output "True"
}


I think that $notificationDate -eq $today makes the logic very clear to anyone reading the code.