My friend and partner-in-crime JudoHeroes pointed me to an interesting video someone posted (see below) which highlights a one-handed sode attack which presumably got hansoku-make. It goes on to ask why one-handed sodes by the likes of Soichi Hashimoto (JPN) are not illegal but in fact are given scores (sometimes ippon).
The answer, it seems, is that if uke gets injured, it's hansoku-make; and if uke is not, it's a legitimate attack. While some might argue that that's a stupid way to decide whether a technique is legal or not, it's certainly not arbitrary. In fact, it's very logical. If you do the attack in such a way that it straightens uke's arm and injures him, it's considered an armlock done in conjunction with a throw, which is illegal as in the case of waki-gatame done with a throw. If you enter into the one-handed sode without straightening uke's arm, like how Hashimoto usually does it, it's fine.
It's true that sometime even when you don't meant to straighten uke's arm, it just happens in the course of the attack because of how uke reacts. Tough luck then. Top players like Telma Monteiro (POR) and Avtandili Tchrikishvili (GEO) have been given hansoku-make for that very reason even though they very clearly were not trying to straighten their respective opponent's arm.
I guess that's the hazard of doing the one-handed sode. If uke's arm gets straighten either by your doing or uke's own fault, you still get the hansoku-make. You might not like this rule but what would be worse is if the one-handed sode is banned all together.
It's worth noting that Hashimoto has at least once been given hansoku-make for straightening the arm during a one-handed sode attack but since then he has managed to perfect his entry so that it doesn't hurt his opponent's arm. If you want to do one-handed sode, it's a pretty good idea to study how Hashimoto does it. While you're at it, you can also look at how Elkhan Mammadov (AZE) and Alan Khubetsov (RUS) do it. Both are also experts at this.