I can understand why you have this impression, because there is lots of half-baked information floating about on the internet. A lot of the early 'lossy compression artifacts' shown on the net were actually found to have been caused by other factors in the end.
I own an A7 and A7R, along with a Ricoh GR and 645Z. My Sony files do not show artifacts when pushed and, as a B&W shooter, I am quite hard on the files. You can look
here, where 80% of the images are from the A7 and A7R (along with the Land series). The files are not quite as robust as the 645Z files, sure, but they are more robust than GR files, which sports a sensor shared with other Pentax APS-C bodies.
Sony lossy compressed files can show artifacts in extreme contrast transitions and the only place I have seen these reliably and repeatedly produced is in star trails at night, where there is of course a massively abrupt transition from the etched trail and the black sky. I've shot mine since release and not see problems in my normal usage. I cannot speak for everyone elses experience of course, but I would be surprised if they are different to mine.
In short: I am of the opinion that you can absolutely hammer A7 files in the context of regular photographic output. They may not be the very best out there, but they are close to the top. If your application is in graphical work or extreme non-photo realistic outputs, I cannot comment.