It is not that Eastern men are weak I think it is the opposite and that strength is the quality they like of their men and want to see in other men. It may be other qualities and mindsets they are less keen on in their own local men. If these Eastern Euro women were on a local dating site with fellow Russians/Ukranians etc they likely wouldn't say I want a strong man as they would assume most of the local men already know what is expected of them but they would emphasize what they feel is less represented. Here on this site with this audience, they are focusing on the quality they see lacking. As for why it is lacking, we can debate that but I find it hard not to pin a lot of it on feminism and the ever changing rules on socially acceptable behavior for men. I worry for young boys today, they are being taught virtually every natural instinct they have as boys is wrong, that they're so paralyzed at doing something wrong as to potentially offend or worse they do nothing, it's going to lead to a pandemic of weak ineffectual men if it hasn't already.

As for eastern men on dating sites in the west, I have many Slav friends who are in my city, originally from elsewhere. For the most part yes they do quite well initially, they do stand out for being more manly/aggressive what have you than the average local guy but they don't usually have long relationships unless their attitudes are more congruent with the norms here. If they still harbor ideologies of their home countries they can struggle until they meet somebody like minded. I've found most end up going back to their home countries for spouses.