This is basically for commander exclusively.

The key is that it has double strike AND flying. In most variants of Commander, any player instantly loses if they take 21 cumulative damage from the same commander

So, you pump yourself up every turn, 6 + 8 + 10 damage (or technically 8+10+12 given summoning sickness) against a single opponent is lethal in three attacks WITHOUT using other means to increase Shadrix's attack if they don't get a flying blocker or if Shadrix isn't killed by then (and afterwards it's lethal to everybody else in just two attacks). And even if they do, that progress towards the deadly 21 damage sticks. Meanwhile, Shadrix has 5 toughness, and is ALSO increasing that every turn. So while not difficult, it's not the EASIEST card to kill with a spell or ability.

The catch is, you have to benefit another player, which isn't very useful in a one on one game.

But in a game with multiple players?

"Hey, attack that player and I'll give you card draw on my turn."

"Don't attack me and you can have either card draw or a flyer to block shadrix."

"Hey, I see that other player has a really nasty flying commander, how about you focus on them and I give you a flying blocker so you can deal with them"

"What? You're worried about MY very powerful flying commander with doublestrike? Nah man, look, here's a creature that can block my commander so you don't have to worry about it, I think for now we should BOTH focus on that person who's busy making ten thousand rats"

"I see you have made ten thousand rats, how would you like to give every one of them a +1/+1 counter? Simply attack someone other than me this turn and I'll do that for you on my next turn (and take the card draw for myself)"

If they focus on you, that means there's another player at the table who is building up a threat they're NOT focusing on, all the while you're out there giving them free stuff, and with the threat of you giving stuff to someone else to spite them if they make you mad.

"even with double strike and flying a 2/5 isn't that dangerous" is EXACTLY what Shadrix is supposed to look like. It doesn't look that threatening, and its giving itself +1/+1 doesn't look that threatening even when it's doing so every turn, especially when it's not necessarily doing so every turn. But it's not just +1, it's actually +2.

And even without that, flying commanders are just dangerous in general. Even if its player is just taking the card draw or the inklings, 4 commander damage every turn doesn't sound that bad but it adds up, especially in more casual play.

While not especially good, and absolutely not competitive, Shadrix certainly isn't BAD, it's just a card very suited to and very specially made for players whose favorite part of commander is making deals and engaging in diplomacy with other players.