No it's doesn't always apply in plants.
It's all to do with the number and complexity of chromosomes that the plant has, but plant genetics it's quite a complex subject, as radom mutations and chromosome doubling (which can result in it becoming fertile) can occur much more readily in plants than animals, it can also be forced by the use of chemicals such as colchicine.
Some species yield fertile hybrids with certain species, and sterile hybrids with other species, others will be fertile if one plant is always used as the pollen parent, and the other always the seed parent. The plant with the lower chromosome count is generally used as the seed parent. The reason for this isn't yet clear.
Simply, there are usually two types of Hybrids
1. Interspecific - in other words, both plants from the same species such as a loganberry (Raspberry x Blackberry) all plants are in the Rubus species. These are more likely to be fertile than not. Sometimes naturally occuring when two plants are grown in close proximity.
2. Intergeneric - 2 plants from different genera. eg. This gets really interesting in orchids as they are breeding from 2-4 genera together so you get new names created such as Vuylstekeara which is a man-made species made up from 3 other plants from Cochlioda x Miltonia x Odontoglossum - a trigeneric hybrid. The intergeneric hybrids are far more likely to be sterile than interspecific.
Heucherella is an intergeneric cross and it does appear that all the resulting plants are sterile.
Hope this hasn't confused you more.
