Conversions... what's the problem?

the main PRACTICAL disadvantage is that the bottom bracket will be lower —*meaning you'll be more likely to get pedal strike. The solution is to use short cranks, but you'll get less leverage and so it will be harder work. e

About that short crank solution, quite honestly I don't know how a mere 5mm shorter would make a huge difference in preventing pedal strike, I mean 5mm, that's pretty short.

I'd have though it come down to the choice of pedal than the crank length itself, a short clipless pedal would have less chance of a pedal strike than a wide BMX-style pedal.