What it appears that this site has done it get all the available war records and put them on their site. And then charge for the privilege of using their site.
They have NOTHING that cannot be found on others sites.
So, as for WW1, if Ancestry etc., does not have it, it does not exist.
We have always been told that only 30% of WW1 records survived WW2. A lady from the National Archives was on telly about 4 weeks ago and she actually confirmed that it is only about a 25% survival rate for these records.
This is the problem with military records and the amazing marketing Ancestry have done and so most people believe this common misconception that if it's not on Ancestry then it isn't anywhere which simply isn't true.
I would recommend you take a look at each sites collections list, you will see practically every site has exclusive records not available on any other site, it is of course an obvious business tactic.
There are always obscure collections that the sites transcribe from time to time, otherwise why are we are still subscribing hoping for new info to come to light if all the collections have already been done, and yet just about every site sends us regular emails telling us they've just added new collections!
There's lots of collections outside Ancestry worth looking at, whether your ancestor is in any of them is another matter of course.