I suspect some of my experience is now out of date, as it's now spread out over quite some time. The most recent issue I had to deal with was Chrome: when drawing the canvas at high-res it was being a bit slow at blitting some bitmaps and so was running at 30fps. Something is tied to the framerate with the pointer events implementation and so the events also lagged behind, which made drawing on the canvas quite difficult as the display was 250-500ms behind the user. Eventually 'fixed' by turning the resolution down, but it was a real pain finding what part of the application had got behind (FlowBetween being designed not to lag but to catch up when the display can't keep up). That's quite a subtle one and the pointer events lagging is easily mistaken for the frame rate lagging.

Other browsers don't do this, but they've had a few other issues: what I remember in particular - some only support pressure information using the touch API, and some seemed to support pressure information on different APIs on different platforms, so both pointer events and touch events were needed.

All of these are maturity issues rather than real problems with the API, though and I haven't re-checked some of the older issues recently - that Chrome issue was still happening back in January so might still be around, but the others I last encountered over a year ago so may have been fixed by now.