I tried to get an assembly quotation from Aisler but they can't deal with Excel files + Gerbers and the support is useless. Smells like amateur hour to me.
Well, you get what you pay for. You can't expect state of the art service, quick turnaround and prices of China. If you provide just gerbers and excel, there has to be a CAM operator putting this together. This is easily 80-100 Euro/hour. That's how much a qualified personnel costs in Europe.
How is that any different compared to using an Altium or Kicad file? One way or another they need to convert the output into Gerbers (or any other photoplot format), pick & place data and bill of materials. I'd say loading a file into Altium and getting the production files from it is more work compared to getting the ready-to-go production data. For example: Eurocircuits, Seeed and JLCPCB have no problems with using a bunch of Gerbers and drill file to produce a PCB correctly. Aisler has no idea what to do with the exact same dataset!
I have professional experience with BetaLayout. They are not going to touch assembly unless I provide them data in the exactly same format as they want. And by "exactly" I mean even columns and rows in excel. I remember this because I had to re-upload data for some of my projects before.
And the difference is big. Both KiCad and Altium are known formats and they can export exactly what they want with a single click. Or better with executing a single script. High level of automation is the reason why Aisler is cheap (considering Europe).
Also, if you want electrical test, you need to upload the PCB file (at least that's what I was told) - again, it is about automating the PCB manufacturing process.
Aisler has definitely some gaps and room for improvement. Nevertheless, I've been using their services since the beginning for both - professional and hobby use. I get PCBs in typ. 5 working days shipped from Europe for very affordable price. Additionally, Aisler supports KiCad.
What else should I want? I rarely shop outside of Europe. I value Europe, environmental protection, human rights and 40-hours work week.