I was initially un-hyped over this because it's Valve. I expect their console experience to be super janky. Let's be honest, everything Valve touch eventually 'works' but there's always some weird shit that goes along with it. The more videos I watch though, the more I'm warming to the Steam Deck.
I'm not one for pre-ordering the first run of anything by Valve but I'm interested to see it in action and look forward to peoples posts on the forum. I'm especially curious about performance and input latency of the Proton layer on a low power AMD APU. Historically, and from personal experience, AMD GPU's on Linux have been pretty bad.
As for the SD card thing: you'll probably be able to move games between storage devices. You wont have to stream problematic games from the SDcard.
Given the nature of Steam's DB and filters, it should be possible for users to flag games that are problematic via SD, and label them as such. The OS could then tap into these recommendations and ask you if you want to copy it to internal storage before you play it. At least it's something I'd consider if I was working on it.