The amount of solder that the process is capable of applying is the biggest limitation. I believe that solder paste is only approx. 50% metal by volume. When connectors are soldered in intrusive reflow, often the stencil is "overprinted" to get more solder. In certain applications such as multi-row (3 or 4 rows)or if the connector is very close to another component, overprinting of the stencil is not an option. The connector lead pitch is also a very important factor in the intrusive reflow process.
Solder preforms or solder flux bearing connectors will deliver more solder volume than the intrusive reflow process.