Hipmunk was the absolute best, I miss it everytime I have to search for a flight.
Not sure I would put it as strongly as that.
Hipmunk was OK, one of "the better" free tools out there.
But I'm afraid it never had anything on ITA Matrix. ITA always had copious amounts of extra special sauce.
I extensively tried all competitors I could find (including Hipmunk) on a variety of real-world scenarios. Nothing ever came close to ITA and I was very worried indeed for its future when Google bought it.
Hipmunk actually let you use ITA syntax to search for very specific route constructions, which you could then buy.
If that doesn't work and you really want a specific itinerary, I've had good luck with amex travel concierge, where I provided them with copy-paste of ITA matrix and they made the booking.
I booked tons of tickets over the years using ITA as a basis.
Its not that difficult. Get the itinerary finalised in ITA and then either:
(a) Head over to the appropriate airline's website and enter in the legs; or
(b) Call up the appropriate airline's reservations callcentre
The savings achievable via ITA far outweighed the minor inconvenience of double-keying.
But calling a travel agent has always worked.
p.s. Google seems to be the worst at bait and switch, but that's different story.
But ITA was so comprehensive you could find loopholes, like when you add an additional destination and your first class ticket drops in price by 50% etc. It is super missed in the mileage community!
Also looks like they're building a successor to Hipmunk called Flight Penguin: https://flightpenguin.com/
[and yes… that was why we went with a sub model over pay per use]