Offspring formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?

Yes, it's true. Sexual reproduction is characterized by meiosis and fertilization which in turn add new genetic combination (i.e. new characters) to the existing species. Genetic diversity serves as raw material for natural selection which favours the characters that impart a survival advantage to individuals. Accumulation of these adaptive characters over generation makes the population better adapted to prevailing condition and imparts survival advantage. It is not always true, for e.g. take the case in which two parents are heterozygous for a specific trait/allele (suppose sickle-cell anaemia). The parents are not affected because they still have one good allele. However, if their offspring acquires the defective alleles from both the parents (i.e. homozygous for the sickle cell anaemia allele) then the offspring will have a very short lifespan. Hence, it is not necessary that an offspring formed due to sexual reproduction will have better chances of survival.