I used to commute on the Tube, though now use it far less frequently. I have never had an onboard check, ever, though I did see plain clothes revenue staff walking down the platform to get on a train at Moorgate not long ago. I also had someone at the barriers check my Railcard (when I had one) after touching in at Kings Cross St Pancras, which would have been about 5 years ago. Barrier checks do seem more common, and as above, they can be easy to spot if you know what you're looking for.
I do tend to critique onboard revenue operations on trains of all sorts, largely because I've generally been there and done it, and my opinion still stands that I prefer the visible deterrent and regular checks of all passengers on the Trams and DLR. The FirstGroup Tram ticket checks are usually particularly well operated, and they tend to adopt a fair approach to people experiencing difficulties with tickets, as well as being able to get people to co-operate even on a crowded, late-running tram at somewhere like Mitcham - quite a skill, really.
Intelligence-led checks are all very well, but there are always surprises to be found, and people who'd otherwise slip under the radar can often be found in routine checks of all travellers. There are also ways to quietly note people for further investigation or monitoring during broader checks - intel has to be gathered somewhere, and this can be one way.