The Thalmor is strong because they ruined the Empire in the last war, and the Empire is now struggling to survive with its own problems(like the insurrection in Skyrim by the Stormcloaks) while the Thalmor is rebuilding its armies and economy while spreading their influence throughout the world.
You probably won't see any major military campaigns by the Thalmor for many, many years and so don't expect to see a 'Thalmor invasion' anytime soon.
They go by 'divide and conquer'. They'll probably wait patiently around, rebuilding their army while sowing distrust, rivarly and chaos in Tamriel and pick them off piece by piece.
Also, the reason everyone hates them is because the Thalmor are a bad thing for anyone who's not an Altmer. Even worse than Ulfric and his way of treating any Nord(Ulfric only frowns at anyone who isn't a nord, but the Thalmor actively seeks the extinction of anyone who's not an Altmer)
Personally, I love them IRL, I think they're a GREAT faction, but of course my non Altmer characters hate them RP wise
