Do you have experience with high voltage resistors THT?

[Miyuki]

I find Vishay have VR37 series High Voltage Metal Glaze Leaded Resistors
Are really cheap and are in stock (what is also important these days)
They have  Umax. = 3500 VDC compared to common few hundred

I want them mainly for mains ZC detection to MCU
Is there any trap or I can sleep well with them?

[TimFox]

The maximum voltage rating is good, so long as you also obey the maximum power rating:  3500 V at 100 k (minimum available resistance in this series) is 122 W, but the power rating is only 0.5 W. 
Above 25 megohms, the voltage rating dominates over the power rating, but the series only goes to 33 megohms.
Also note the 700 V insulation rating (through the coating around the resistor to anything touching the surface).

[bdunham7]

I've been using the very similar HVR37 series in applications like HV dividers and such even when they might be 'overkill' because I'm more comfortable with the specs,.  In mains-sensing, this would make me more comfortable as well, especially the 10kV pulse withstand..  They also seem to have a low voltage coefficient.  The differences between VR37 and HVR37 seem slight, the HVR37 description refers to a flame-resistant lacquer and they are a bit bigger.  They both have the 700V insulation limit (never noticed that before) but hopefully your PCB design doesn't run traces under your HV resistors or anything like that.  Where I use them I have thousands of volts so it becomes obvious you have to keep stuff away.  Where mains are concerned, you should treat it as if it were thousands of volts--because it might be someday!  So VR37 is probably a much better choice than something with a 300-500V rating.

[Miyuki]

230/400V mains have a 2.5kV surge level (what shall every device withstand) so it is still in continuous rating, which is nice
I can see why they have just 700V insulation limit. That enamel coating is easy to damage. And every little scratch can easily compromise it when another component touches.
But I plan to have them far from everything, no traces near

[Benta]

I've used Vishay VR37 and VR68 for mains applications with good results. Quality and reliable, absolutely no issues.

[sandalcandal]

Have used 80x10Meg HVR37 resistors to make a DIY EHT probe based on the April 2010 Silicon Chip magazine article. Was going to pot it but never did. Seems to work fine for >30kV.

Also started but did not finish a HV oscilloscope probe project using HVR37 resistors potted in paraffin wax. I don't recall breakdown being an issue.

[BrokenYugo]

I rebuilt the HV divider in an old oscilloscope with VR68 resistors and it seems to work, given those are 10 kV resistors with like 1200 volts across the whole chain.

I don't think there's a trap with these like the SMD parts that require potting to meet their HV specs.

[CaptDon]

We built high voltage ion sources for the 'anti-static' ionizers used in electronic manufacturing.
We had a constant problem of arcing even though we used potting and overkill on physical
spacing. Lesson learned...... DO NOT touch the bodies of those resistors with bare fingers!!!!!
The contaminants in body oils will fluidize in the potting and come back to haunt you!!! We
had dozens of these devices returned from the field. We were a contract manufacturer for
the anti-static fan devices. Employees who eat potato chips for lunch and then return to the
assembly area without washing hands, wearing gloves or finger cots cost us a bunch of money
as a manufacturer!!! You can't watch every monkey in every show!!