Apologies for this late response; I'm currently traveling.
Before my 2024 trip to several Balkan countries, I researched Airalo's coverage in the area. As I recall, there were one or two countries with no service--maybe Bosnia and North Macedonia. I ended up using country-specific Airalo eSIMs where available and my regular, pricey Google Fi SIM for the others. I don't remember issues with Airalo where I expected to use it
From an earlier post on this forum I learned about Bcengi and decided to use it both at home and when traveling. Its per-GB rates vary by country but for the places I checked in Europe, it seems less expensive than Airalo and vastly cheaper than Fi's $10/GB rate. Bcengi's ESIM has the advantage of having (pretty much) worldwide coverage and no time or usage limits. You make an initial deposit and when that money runs out, the company charges your credit card again. I found initial activation very easy and used Bcengi at home for several months without difficulty.
When I bought a new phone I ran into a bit of an issue, but the Support folks were extremely responsive. They issued a new smart code so I could install the eSIM on my new phone and transferred my remaining balance. I found I had to turn on Data Roaming, which I guess defaults to "Off" on new phones. I also needed to turn off "Select Automatically" under Network Operators and register the phone on the local network (3 found, but I think only one of them works with Bcengi), which I hadn't had to do during my initial installation. I don't know whether that's related to the new phone OS or a change Bcengi has made.
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I'm now wrapping up a three-week trip to Bosnia. Bcengi has worked fine in Sarajevo and Trebinje. I don't recall problems in Travnik or Jajce, but I had little need for cellular service in those small towns so I can't be sure. I had service during much of the time on buses from Sarajevo to Travnik and on to Jajce, Bosnia is a mountainous country, so it wouldn't be unexpected to encounter some service drop-outs in rural areas. However, I was very surprised the Bcengi eSIM didn't connect in Banja Luka or Mostar. I don't know whether that was a short-term service problem (those were consecutive destinations for me), but I cannot believe those are coverage dead zones for Bcengi and the local network operator it contracts with. I reported the problem in Banja Luka but haven't had a response from Bcengi Support.
Despite the issues in Banja Luka and Mostar, I'm happy to continue using Bcengi. I do have the advantage of Google Fi (expensive though it is) as a back-up option. And if Bcengi doesn't work in some places, at least the money you've spent won't evaporate at the end of a fixed service period.