First off - you don't need to inject the carnitine. L-carnitine is almost 100% bioavailable when taken orally. Just use an oral L-carnitine supplement.
Based on experience of myself and 17 or 18 clients of mine I'd most certainly have to disagree with your statement.
In my experience the oral L-carnitine is essentially useless (tried many many different brands between us) and the injectable L-carnitine was night and day difference when it came to lipolysis.
Infact, from the results I got, Id rank it in the same ballpark as a very low dosed clenbuterol. And if you take side effects into the equation then injectable L-carnitine might be #1 fat burner out there.
Of course the downfalls are the injections themselves....the volume of injections tends to cause more pain...and lastly because its fairly new to the market, there are very few companies producing the product. I know of three companies producing injectable L-carnitine for HUMAN consumption...and of course there are quite a few which are veterinary use. Which seems a tad shady. But, it has given the exact same results as the "human grade" stuff. Keep in mind though, anything made for a race horse is likely not junk. As there are some race horses worth millions of dollars...and even your run of the mill race horse is worth as much as a new Cadillac. So they wont be selling absolute junk product with super high heavy metal content to these super expensive animals.
However with that said, I will always try to lean towards the product that is actually made for a human being. Lol. Call me crazy.
Lastly, regarding cardarine. I have seen some amazing results from cardarine and am jealous that I havent used it myself just yet. Im greatly looking forward to doing so.