I've just taken out my second three year service plan whch also includes 3 MOT's. In my opinion, it's well worth the money. Means the cost of scheduled services are fixed for three years and therefore not subject to price increases. Also the same for the MOT's.
Should items require attention that aren't included in a normal service, such as brake pads, brake discs, worn track rod ends, etc, YES there will be additional costs. Filters and fluids will be covered in the service plan. Also you get the benefit of the breakdown cover by having the car serviced under the Hyundai Essentials Service plan. Home start, Roadside assistance, Recovery to a garage within 10 miles of the breakdown.
One potential drawback is paying up front to get the best price. Can make a dent in your finances if you're not prepared for it. Yes you can pay by monthly instalments but as with most instalment plans, there is an additional cost.