Personally I feel great on orals, even very high dosages, EXCEPT for the massive heartburn that ruins everything. Injectable, if doing high dose, just make me feel bogged down and overloaded or "toxic" over time. My theory on that is that they build up in the system, whereas with orals, overnight you are at "zero" again lol.
Orals affect HDL very negatively as was said here, it has something to do with the liver. They can be done, by some, without totally tanking HDL, dependscon dose, compound etc. However, injectables can also reduce HDL drastically. And as dj said, some of it has to do with estrogen, so now many bodybuilders do little, some even no, aromatase inhibitors and are worried all their years on AIs may have done damage.
Regarding metformin turning Type 2 to Type 1, never read about that at all, but would like to if someone linked me. One possible mechanism that immediately pops into mind if the patients had very poor glucose management DESPITE the MET, that would mean they destroyed their remaining working beta cells in their pancreas. In that case it would not be the MET that is to blame. This is why some practitioners have advocated to put a bad case of Type 2 immediately on insulin to get immediate control of the glucose. If then the patient loses a lot of weight, exercises and so on, he may be able to get off the insulin. High glucose is why bodybuilders on high GH get on metformin, but some find it's not enough and add for example Lantus at night so their nighttime and morning glucose stays in range. Now taking insulin can further hurt insulin sensitivity but it's way preferable than the alternative where you actually start damaging tissues, and the pancreas itself. SO, the insulin is actually protective. If and when you stop the GH you stop the insulin as well. That said, some bodybuiilders can use A LOT of insulin without getting resistant. High doses of basal insulin are bad for sensitivity over time e.g. Lantus, I'm talking of say 100iu plus every day, many do just 10-20iu to assist the pancreas.
One can do both MET and some insulin if he is very careful.
I don't like that my replies get so long, but I can't fit a full explanation into a sentence or two