There aren't many. Not all coming from your beloved guy, either.
People will question you at all turns: "Is he really a guy if he wasn't born with a dick hanging between his legs?!" Biological essentialism that has wracked my brains ever since I found out that men were not "allowed" to be feminist in the old days because one could only understand and feel indignant about women's rights or the lack thereof if one was born woman has come about to haunt me again: the biological essentialism "you can only fight for trans rights if you're one". The fuck you can't. Was slavery abolished by enslaved black people? No, for the simple reason that the agency of the black people was never recognised during the time of slavery. So who abolished the slavery? Anyone, but a black person. I will take anyone fighting and working for trans rights at this time, be they trans or not, and be thankful for all the non-trans people who are in the movement for us, the trans folk. It's high time I made another entry altogether about this bio essentialism in the trans movement because it's been pissing me off on a scale one can't imagine. To get back at the issue: if the lack of a birth dick maketh one less of a man, how would you define a man who had unfortunately lost his dick or balls in, say, accident? He's still a man, no?! So explain it this way: your beloved dude had a birth accident, which rendered him dickless, but nevertheless a man. People will claim: "But your relationship is essentially a lesbian (or straight, if your guy's gay) relationship because he's lacking the penis the holy grail". To fuck with that. How a transguy feels about himself is the only, single-most important reality. It's easier if your guy has had some HRT and other medical interventions, but even if he hasn't, he's still a man, even if he appears and sounds differently from most other guys. And no, the relationship is never lesbian, or straight, because your guy and you know what it is. Unfortunately or fortunately I had been in many a "lesbian" relationships, but in my head they were never lesbian relationships. I was me, someone who was trying his hardest to be a woman, hating the fact that I was born in this body, but nonetheless loving it on other people. That was that. Although things may look like something on the outside, it may very well be something different altogether to people in the relationship.
People will say: "But what about children? You can never have your own children with a transguy!" You know what, a few months ago I was shocked speechless about a friend saying something to the effect that she didn't give birth to her child, that therefore that the child was not really hers. WTF?! Having been in the queer community since the age of 20, I know that family is not always blood. I've seen truest families made up of people not connected to each other by blood. Plus, in this age of advanced medical technology, if one can afford, one can absolutely have a child with a transguy. As can a couple of lesbians. Or a gay male couple. With the genetic imprints contributed by both the parents. If one can afford, that is, which is a big issue. If not, then at least by one parent. And there is always adoption. Personally, I've always thought it was a little too selfish for people to long for the genetic progeny when so many children on this planet are growing up without parental love, in horrible orphanages, or on the streets. I had always wanted to adopt rather than have my ancestral diseases (high blood pressure, cardio-vascular abnormalities, skin disorders, high cancer predisposition) passed onto my direct progeny. So thanks, but no thanks, I'd rather adopt a child and see him/her grow up to be a great human being than insist on them having my genetic imprint.
One and only disadvantage that I can think of that could turn out to be a real issue is the fact of medical interventions that will continue throughout his life, ending up being quite very expensive in the long run (which, again, may not as it really depends on where you are as some countries do cover transition-related medical interventions while some don't). Of course, there are trans people who choose no or very minimal intervention, again, it all depends on your guy, his body dysphoria level. If your guy really wants all those medical interventions, but at the same time can't have them, you will also end up dealing with the hard, hard, and very ugly end of the stick: his deep, deep depression and all that comes with it. Again, everyone's case is going to be different.
There could be many more, these are just off my fuzzed brains at 1am in the morning. Feel free to add to the list. Signing off.