For start Ukraine treaty does not handle subsidies. So France and other countries would be able to 'dump' their subsidized agricaltural product on Ukranien market. Ukraine farmers do not get subsidy, they have higher prices and would eventually bankrupt.

Industry is concetrated on east and exports/imports mainly to Russia. EU would make it very hard to trade with Russia. Factories could not sell product and could not import materials.

Joining EU is long process which takes years of negotiation. Poland is good example. But just signing whatever EU throws your direction is very bad deal.