Any kind of convective cloud is bad news for structural ice, but the by far worst ice encounter I have ever had was in NS, base 1500ft and smooth, tops 4000ft and smooth, all very smooth with zero turbulence, SFC temp +3C, temp in cloud at 3700ft -5C, 30mm of clear+rime collected in 5-10 mins. Max engine power (TKS deiced prop) i.e. 250HP and could not climb. Vs rose to about 100kt and that was also the max speed achievable in level flight.
I got out of there fairly quick
but I was content to give it a go for the extra few mins after I spotted it (I watch for ice constantly in IMC below 0C, obviously) because I knew the easy escape route was by descent.
No control problems provided one kept the speed up, preferably in a descent at 120kt
That established the absolute limit for the aircraft.
Interestingly, climb or descent through the same layer, 1000fpm, collected under 5mm, which hardly matters on the TB.
Most pilots will tell of similar stories, but not openly.
Of course nobody has ever done it in a PA28... and 1mm will definitely kill you
Ice is very hazardous but knowledge is better than ignorance or blind fear.