i prefer silk thread for applique- it is so fine/strong and comes in perfect colors to disappear- a light gray on lights, or a darker gray on darks and you never find it-
i read that nylon invisible thread is the one that gets (pokey) and brittle with time- will not hold up to any heat- as in iron or dryer- and is not a thread that should be used for laundered items---but i think there are different opinions just like with everything else-
check out one of the thread company sites (like Superior Threads) which offer free information sheets on the different threads, wts, properties and what needles to use with each one- it is invaluable information when deciding the best for your project