Historical Linguistics if one of the more brutal subfields in Linguistics, probably because you can hardly prove much of anything.
It involves reconstruction of dead languages or earlier aspects of existing languages. The dead languages have left no record and are often 7-10,000 years old. The earlier phases or existing tongues also have left no record.
So it is unprovable guesswork guessing at what ancient languages looked like, with no real way to prove if anyone is right or wrong because the languages no longer exist.
On top of that, the field has become mired in stick in the mud conservatism such that I doubt if any new ancient language families are going to proven in my lifetime. The conservatives keep moving the goalposts, and no evidence is ever good enough.
Being a Historical Linguistics conservative is the hip and cool thing to be in Linguistics, and the peer pressure in the field is worse than an eighth grade playground. If you take a liberal position that says that some ancient language family like Altaic exists, the peer pressure on you as a fraud, idiot, kook, crank and loser is unbelievable. I am amazed that there are any liberals left promoting daring new ideas on ancient language families.