Uhm, maybe I am missing something but what makes you think that SC injections don't require the solution to be free of particles and contaminations? It is never okay to introduce particulate matter into your body and/or things that can cause an abcess if you do not filter. The particles could be a place where an infection can start more easily.
Generally IM shots are a little more acute and effective but less prolonged because it takes a bit longer for a drug to disperse in the hypodermis. Also you can only do smaller shot volumes via SC.
When I took my K via IM for a while, I had long enough needles for IM but I did not really trust shoving all of it in, seemed counterintuitive that that would be necessary to get into the muscle. Also during that time I was a skinny guy (I still do not have a lot of fat, but I am definitely in better shape). Not sure how many of the shots were actually IM for me.
I got this feeling of having hard lumpy knots where I injected a lot, I assume those are scars within the muscle? Nothing serious, but I just assume that I did not cycle spots enough.
Within a few days I will once again be taking IM shots, with other drugs now.
I will aim for actual IM shots, not SC. Not that they would be so bad but I would rather have a more effective result and no blob of liquid sitting around in my skin, does not seem so ideal.
And like I said: you still need to be hygienic.
Spot selection with SC is probably a little different since you are not avoiding the same things, but since you shouldn't go TOO superficial and it is not always obvious how thick the dermis and hypodermis layers are (or is it?), I would not attempt doing SC at too random a place. Not exactly sure what complications there are, or what things you can hit where, but just stick to the same sites as IM - why risk anything because you can't be bothered to do this properly and are not aware of all the ins and outs?