Flying US to Ljubljana

I had no problem with Delta/Air France from Pittsburgh to Ljubljana. I had about 3 hours in Paris which was more than enough time even after arriving a little behind schedule. The connection from Paris was at the G terminal (I believe) which was a bit of a trek from where my US flight landed but getting through immigration / security was no issue (I was able to use the automated lines for my passport check) and the terminal for my connecting flight to Ljubljana had Starbucks, places to eat, shop and a large sitting area.

Maybe look at those two as they are airline partners so any checked bags will be put all the way through and once you arrive in Slovenia if you have nothing to declare at customs you just walk right out.

Buy the cheapest fare that meets your criteria. Something can go wrong with any carrier. For general comfort i prefer Turkish Air but not if it's more than 5% more.

We flew United/Lufthansa last month, and had the exact problem that you described. I think we're done with that combination.
When we flew to Ljubljana from Phoenix in December 2023, we flew Delta/KLM/Transavia (a KLM subsidiary I believe). Service on Transavia was below that of the other 2, but travel on the 3 was seamless.

It's been a while, but we flew Lufthansa Seattle to Frankfurt, and Lufthansa to Ljubljana. I believe currently the only option from Frankfurt to Ljubljana is Lufthansa. I'm not a huge fan of Lufthansa but they (usually) get the job done.

About booking separate itineraries: I would try hard NOT to book two separate tickets. If you are on two tickets, you risk problems if your inbound flight is late (having to buy a new, same-day ticket, which can be shockingly expensive). While it's a short flight and there are rail options, misconnecting would be a big headache, not something I welcome at the beginning of a long trip.

If you really want to buy separate tickets, allow LOTS of time for the connection. I would want at least 4-5 hours.

Personally, I'd try hard to book it on a single ticket. You didn't mention where you are coming from, but Lufthansa is a huge airline and has many huge partners (eg United), and I see no reason to avoid them. I would not say that the integration between United and Lufthansa is "broken" (not in my experience). I fly United quite often and while they're not perfect (what airline is?), I think they're good enough (same goes for Lufthansa).

If I was flying to Ljubljana this summer, I'd probably be on United and/or Lufthansa, and I wouldn't be concerned about it (any more than I'm concerned about flying any major airline) - although I'd be tempted to try going via Istanbul on Turkish Airlines if I could.

If you can book this as one reservation do so. If there is a problem with the first flight and you miss your connection, the airline has to get you on the next available flight at no cost to you. You might get a voucher for free food or even compensation.

If you buy a separate ticket for the intra-European segment and you miss that, you are out that ticket and will have to buy a new one. Last minute tickets are not cheap.

I didn't say it, but yes, one ticket.

We fly out of Boston and have gone in and out a LJU a number of times with no issues. We flew there last month on Lufthansa with no problems. Boston to Munich to LJU and returned LJU to Frankfurt to Boston, all on one ticket. I was concerned as the Frankfurt layover was under two hours, as was the Munich layover, and there were no choices for longer layovers but each trip was seamless.

We like to fly British Airlines since they fly direct to London from PDX (Portland, OR) and then we flight to our destination in Europe. This June we are doing this combo to Ljubljana

We are actually flying Delta/Air France from Atlanta to Paris to Ljubljana with a 3 1/2 hour layover on August 29(for the RS Best of the Adriatic starting August 31st). We prefer Delta and their partners since we go out of Atlanta. This was the best option we found for as going out as we prefer to get to Europe for the layover in case things go wrong. Multiple options once you get to Europe if you miss your connection.

I flew Delta from KC to Detroit to Paris and then had a 4-5 hr layover there and caught HOP (which was part of the same Delta reservation) for a quick 2 hr flight to Ljubljana. It was great. Ljubljana airport is small but I pre booked transportation to my hotel with GoOpti for 28 Euros and they were waiting for me when I got there.