Does using a fast track airport service guarantee a hassle-free entry? Or is it still possible to be taken away for questioning if you have an extensive visa history?

Fast track only provides smaller (faster) queues, not faster processing by IOs. If they feel like hassling  you, they will.

I must have flown into Thailand 100+ times over the last 12-13 years.  Mostly on Emirates. Sometimes in Business. Sometimes on other airlines from China and other SE Asian countries. Bizarrely the only time I got asked about finances was Fast Track from a Emirates flight (Business Class ticket) in 2019.  The male IO started asking how I was going to finance my visit and how much cash I'd got on me.  I wasn't exactly sure because I had notes of various currencies. I guess he was looking for ฿20k equivalent.  Anyway I showed him a handful of GBP, USD, CNY and THB and he was satisfied.  He was polite, but it seemed very odd. 

"Anyone been grilled at immigration when using airport fast track?"

 

Never.

 

 

2 hours ago, JoseThailand said:

Does using a fast track airport service guarantee a hassle-free entry?

No guarantee.

 

Several times at fast track I was questioned why I was in Thailand so long. Was on 2 tourist visas in between my non O visas. 

I would argue that with an extensive visa history you may even be more likely to be grilled at fast track.

IO's there are far less busy, and would have all the time in the world to scrutinise your history.

 

The reality is that it's going to be a lottery when coming back through Bkk.

 

What visa will you be coming back in on? What's your history?

If ya are entering the country with a non-suspect, and maybe shady visa history they never bat an eyelid. The solution is not to try to 'work the system' and this wont happen to you, or anyone else.

2 hours ago, mockingbird said:

What visa will you be coming back in on? What's your history?

3 years in Thailand on tourist visas, covid extensions and border runs.

3 hours ago, JoseThailand said:

3 years in Thailand on tourist visas, covid extensions and border runs.

Yeah, that's a lot. Don't attempt to fly in on a visa exemption. I haven't read of anyone post Covid being denied entry on a tourist visa. To really mitigate the dangers of denial, either spend some time outside of Thailand, or maybe look at other safer entry points.