My DDad advised me not to - privately and once only - for a very specific reason. FWIW, he wasn't wrong. He liked and respected by DH-to-be and once he expressed his feeling to me, he offered as much love and support to both of us as possible for the remainder of his life. Similarly, I've advised two different couples, also for very specific reasons. They both went on to marry and I didn't ever begrudge them not taking my advice. For me, it was more a matter of not being able to live with myself if I hadn't at least tried. We're all still close and both couples are still married but the concerns I raised have certainly manifested themselves in their lives, just like my DDad's did in mine.