you're coming a bit late with this and I don't have the patience to repeat everything. But here's one example that would give you an idea about the 'cons', as opposed to the 'pros' (because there are pros too), that foreigners cause:

Tighthead prop: long, laborious training, and very rare to find a good one anyways. Just buy a good one who's already all trained and ready to go from Georgia or Samoa. No time to lose. Our French Tightheads never get the attention they need, playing time...etc...whole league is full of non-French Tightheads...country is in a Tighthead deficiency situation. French scrum suffers. Clubs are taking shortcuts, unnatural shortcuts, hiring all these internationals. Foreigners are there to fill gaps and add a dynamic to a group, but shouldn't be the fabric itself you need to build teams with.

It's artificial, a huge shortcut, and a problem for local talent.