Solid, efficient non-porous ice.
Despite what the numbers say it's really 4 for a 3 strength barrier, not 2 strength since the +1 strength applies to itself as well. 4 for 3 strength, 1 sub isn't amazing, the neutral Bastion is 4 for 4 strength with the same 1 sub.
Seidr Adaptive Barrier gains strength the more ice is in a server. With 2 pieces of ice it becomes 4 strength, equivalent to Bastion. With 3 pieces of ice it becomes 5 strength.
Taking the generally efficient Corroder as a base to compare against Seidr is 2 to break in a 1 ice server (boost 1, break 1), 3 in a two ice server, 4 in a three ice server, etc. Note that the popular Eli 1.0 is 4 to break with Corroder as well.
Eli is one credit cheaper to rez but has two disadvantages compared to Seidr: you can click through it and it's on the MWL. On the other hand Eli has a consistent tax while Seidr depends on how much ice is in front of the server.
My gut feeling is that at 3 pieces of ice Eli and Seidr are about equivalent, all things considered. The tax is the same but Eli is 1 cheaper. Seidr on the other hand doesn't cost extra influence due to the MWL. At less than 3 pieces Eli has the upper hand while at more than 3 pieces Seidr is better.
Comparing to other pieces of ice Seidr has a cost consistent with it's closest relatives (1 ETR sub barriers). A Wall of Static is 3/3, A Bastion is 4/4. Rainbow is 3/4 but can be broken with any breaker.
Whether the strength gaining ability of Seidr outweighs its initial disadvantage of being a 4/3 in a one ice server depends on the amount of ice you expect to put in front of a server. From my experience R&D, HQ and a scoring server often have 3 pieces of ice in front of them around the mid part of the game. More than that is less common though by no means rare. As I said before I consider Seidr to be comparable to Eli 1.0 at 3 pieces of ice, taking into account the MWL. In fact I will go so far to say that Seidr is a strong contender as an Eli 1.0 replacement in many decks once Eli 1.0 rotates out.
At 3 ice Seidr is solid. But it's when you go beyond that, into clear glacier builds, that Seidr really shines. From 4 ice onward Seidr becomes a really efficient, and at 1 influence it's a good choice for a glacier deck of any color.
Seidr is in HB and I think it's quite fitting. It's Efficient from a cost/tax perspective. It's Reliable as you can't just click through it. And it's Humane since it doesn't try to kill the runner.