I think you have to separate the legal (can I take off/land in the required distance in the reported conditions) from the practical, i.e. would I like to have a decent chance of staying on the runway?
Having had an 'interesting' experience with contamination early on in my flying career, I treat any sort of standing water/slush/snow/ice with a lot of respect and have a deep distrust of observations pertaining to it, especially when precipitation is ongoing.
The takeoff case is the easier one, IMHO, in that a) the aircraft is configured for maximum performance: power, flap, etc. b) the runway you have in front of you is the same length you used for your figures c) the highest risk comes from a RTO near V1: a rare event - at any other time the margins are much greater and d) you have a chance to watch other aircraft taking off and to assess the conditions as you taxi out.
Landing, I would say "all bets are off", unless you've got a decent length of runway to play with (2,500m+) as there are just too many variables. The actual touchdown point is subject to some uncertainty, even with the best technique; the braking action & wind reports may or may not be relevant for the time and place you choose to land; any delay in achieving reverse and/or speedbrake deployment can be critical; the state of your aeroplane in terms of tyre tread depth, etc. makes a big difference and, unlike the takeoff case, you don't need a failure to occur to be in the high risk zone: this is where you are every time you land on a contaminated runway of moderate length... As an aside, I think this is why we're having a rash of runway excursions/overruns at the moment as the exposure is large in this extended period of wintry weather.