Effect on SEO with growing number of subdomains

[Steven87]

Since a few days I'm having some concernes on our website structure regarding SEO.  Since I can't find similar cases I'm curious if the Moz community maybe have a few thoughts on the issue I'm facing

The situation is as follow:

For every new client our company (hosting) receives through www.example.com a new subdomain is created. This subdomain is an backup of the original website of the client and is very much irrelevant to our business. Google can also crawl these subdomains and index them.

Productvariant 1: clientxxx1.productX.example.com
Productvariant 2: clientxxx1.productY.example.com
Productvariant 3: cleintxx10.productZ.example.com

So I think above situation is far from ideal and I think it can cause problems. The problems we could be facing where Im thinking of are:

• no control over content (spam, low quality, bad optimised pages)
• duplicate sites (the backup on our subdomain and the original one of the client)
• impossible to make/manage a property for each subdomain in search console.
• Huge amount of subdomains which could influence crawl/indexation by Google.

Maybe there are some more issues we could face where I didn't think of?

The most common fix would be to use an other domain for the backups like client1.host-example.com and prevent Google from crawling it. This way www.example.com wouldn't be affected.

So my questions basically are:

1. How much will this influence rankings for www.example.com 
2. Are there any similar cases and what effect did it have on rankings/crawl/indexation when it got fixed / didn't got fixed?

[Steven87]

Yes, I'm ware on that one and as we speek it's getting fixed. But I'm curious on x amount of irrelevant subdomains on rankings

[EGOL]

Google can also crawl these subdomains and index them.

Maybe there are some more issues we could face where I didn't think of?

The issues that you are thinking about here are only issues that might impact example.com.

You need to think about issues that impact your clients.  Your clients' domains should have the only copy of their content being indexed by Google.  Allowing their content to be indexed could cause problems for the client.  The duplicate copy could cause pages on their domain to go into the supplemental index.  The duplicate copy could also cause some of their traffic to go to your site.

I would not use a hosting service that allows it.

So my questions basically are:

1. How much will this influence rankings for www.example.com

Just stop this practice of publishing client backups.  Find another backup method.  The impact that you should be worrying about is clients who are aware of this problem finding out about it and complaining loudly.