I have looked into this also. There is a significant cost saving but for myself it is mainly about the time taken. Most schools in the UK specify anything from 6 to 10 weeks to do the IR full time. Many of the schools in Europe seem to be able to consolidate it into a much shorter time. This would obviously not suit everyone but it can be important if you are trying to fit it around work.
I know people that have spent a fortune to get the IR at one of the big named FTO's as they though it would make them more employable. I don't know whether this is the case, I have never heard anyone actually saying that the CAA or any airline do not like Spanish/Polish/Czech IR's and be able to back it up with any proof. I think its more a case of the expensive UK FTO's dont like them as they know they cannot compete.
There are of course bad schools giving inadequate instruction out there both in Europe and the UK. To rule any nations whole pilot training industry as sub-standard is just tosh. This would mean all the nations airline pilots are also unfit to fly.
My personal view is you can pay for the best instruction in the world but if you dont use the IR in anger until the revalidation is due then your skills will have diminished. You can train at a less expensive school but if, once you have the rating you make the effort to fly with more experienced IR pilots, and use it regularly then your skills will improve. I know which type of pilot I would rather be flown by.