Hi Hellie. You very rarely see a push over try in rugby union these days. Their scarcity would imply that it's not too hard to defend against them, or at least to avoid having a try scored from them. However, there are also so many mitigating factors that cause a scrum to disintegrate or be penalised before it gets to the tryline - not just having a weaker scrum. Let's face it, a lot of the time the ref probably isn't 100% sure what is causing it.
Don't forget, a penalty try can only be awarded when there is absolutely NO DOUBT a try would have been scored. Unfortunately, there are so many factors to a scrum collapse that there will always be an element of doubt to whether a try would have been scored. We saw an example of this in the Chiefs v Blues Super Rugby game last weekend. The Blues had an attacking 5m scrum and kept getting awarded penalties and resetting the scrum. The Chiefs even had a man sent to the sin bin because of all the penalties. Why wasn't a penalty try awarded? Probably because despite all the penalties the referee probably wasn't 100% sure it was the Chiefs fault.
As for your question, from a practical point of view, if you have a weaker scrum, it’s probably better to infringe or collapse the scrum and hope for the best than to concede the try. Sounds cynical, but a lot of rugby is like that.