I would recommend to chose an identifier that will not change and is unique and stays unique.
As far as Einstein Recommendations is concerned i dont see a problem here, when your users are atleast a able to understand basic html and very easy amp script. When you insert a einstein recommendation block in emails watch closely to the block that will be inserted there. It will look like this in HTML:
<table>
<tr>
<td>
<a href="https://MID.recs.igodigital.com/rr/v2/5a132e30d647601f19cd6660/l/1/%%emailaddr%%">
<img src="https://MID.recs.igodigital.com/rr/v2/5a132e30d647601f19cd6660/i/1/%%emailaddr%%">
</a>
</td>
<td>
<a href="https://MID.recs.igodigital.com/rr/v2/5a132e30d647601f19cd6660/l/2/%%emailaddr%%">
<img src="https://MID.recs.igodigital.com/rr/v2/5a132e30d647601f19cd6660/i/2/%%emailaddr%%">
</a>
</td>
<td>
<a href="https://MID.recs.igodigital.com/rr/v2/5a132e30d647601f19cd6660/l/3/%%emailaddr%%">
<img src="https://MID.recs.igodigital.com/rr/v2/5a132e30d647601f19cd6660/i/3/%%emailaddr%%">
</a>
</td>
</tr>
</table>
When you send emails with a data extensions attached that has the unique field in it where users have been tracked with it should work. Which would be something like :
<a href="https://MID.recs.igodigital.com/rr/v2/5a132e30d647601f19cd6660/l/3/%%UNIQUE-ID%%">
<img src="https://MID.recs.igodigital.com/rr/v2/5a132e30d647601f19cd6660/i/3/%UNIQUE-ID%%">
Once we created a support-case with the question:
We collect Predicitve Intelligence Data with a customer unique ID and
not with the mail address. If we add Einstein Content to an E-Mail, we cannot change the html-code. We can not change the variable, because it will be emailaddr once again when we leave the edit function of the content block.
Salesforce support itself said:
I went through your last case comments and description and would like
to let you know that you should not use the Predictive content block,
However you should use an html block and copy the code from the PI
email Logic and adjust as necessary.
So we created an HTML block by copying the einstein content and changed the emailaddress to another value which salesforce support commented like this:
Your understanding is correct, the suggested approach is one of the
best suitable way as per your use case.