From the code I read in the most popular stratum-server implementation (stratum-server), it refers to all workers by the username sent, and assigns variables like the difficulty to them.
On your website there is no option to append a unique value like a rig provider id to the username automatically.
So if I rent 20 rigs with a profile and send them to a pool (as I just have), all rigs have the same username and password, and thus the same difficulty. Whether they are 50KH/s or 150MH/s.
This is quite a big issue which can be circumvented by manually choosing a different username and password in the meantime.
But it makes the features like profiles and multibuys you have implemented almost redundant, and may lead to negative reviews when people find the lower spec rigs aren't performing (because their share diff is incorrect), or correspondingly that the large rigs have a share diff too low.
I look forward to hearing from you about this matter and hope it can be resolved quickly.
If you can find him, ahmed who maintains stratum-mining may be able to confirm if this is accurate or not.
Nevertheless, the ability to have a unique identifier for each rig automatically appended would be very useful and ensure this fault cannot happen in any software.
You bring up a good point and a feature we're likely going to implement in the near future should help in your situation.. however this is not 100% accurate.
Most stratum vardiff pools have vardiff controlled at the connection level, the workername is merely a way to track your hashrate at the pool for various 'rigs' you might have connected.
For example, I have 2 7970's that I point at my pool on merc.1, and often rent large amounts of hash and point it to merc.1 as well.. my 7970's difficulty doesnt fluctuate when I do this, and the large rentals get appropriate difficulties for their hashrates as well.
We will likely be implementing a 'templating' feature for worker names, but you will have to ensure that the pool you chose either has the workernames existing already, or auto-adds workernames appropriately.