I understand image processing pretty well. I have a calibrated monitor and use Nikon Capture NX, which incorporates very good noise reduction and sharpening technology.  I know what historgram banding looks like, and color noise, and sharpening halos, and compression artifacts, at 100%.   From that point on it's subjective, and IStock just sees things their way.   Fine then.

istock isn't one person. There isn't someone sitting there who has said "we don't like Jim let's reject all his stuff". There are 100+ inspectors, any of whom could be inspecting any of the images you submit.

If you're getting a lot of rejections that means a lot of trained people with fantastic equipment to spot these things think your photos aren't up to scratch by istock's standards that are applied pretty consistently in my experience. Shouldn't that make you think you need to look inwardly rather than externally and trying to blame to anyone other than yourself for your rejections?

Yes standards are lower elsewhere and it's easier to get images accepted but prices and sales are higher istock because they're proud to offer a higher quality and higher quality QC is a major part of that.

Artifacting can incorporate a lot of things in my experience, sensor spots, compression jaggies, banding... but the critique forum is there for that reason, to help people correct them.

It's hard not to take rejections personally at first, but you shouldn't read the rejection notices as "you suck". If you're willing to put in the extra effort to get images accepted at istock then I personally think it's worth it and there are plenty of people willing to help you too, if you're not then just don't upload to istock.

At the end of the day, it's their game and they can set whatever rules they like but no-one is forcing you to play and it's your choice whether you turn up, play by their rules and reap the rewards or whether you just decide to play elsewhere.