There are other kit makers who compete directly with GR Research. Madisound, CSS, Troels Graveson, HiVi, Parts Express, Falcon Acoustics, and others. Kits skip the retail markup, so can offer higher quality parts for the money. The downsides are that they’re more difficult to hear in person, and have a smaller resale market.
As Kofibaffour mentioned, parts quality is only part of the equation. The implementation and design choices matter a lot too. The kit designs have come a long way in recent years, and some are outstanding. How something sounds to you in your room will always be subjective. Kits offer an easier approach to tweaking that sound if you have the inclination and ability to do so.
I can say from personal experience that many of the speakers I’ve been inside have used fairly poor quality parts and construction methods....especially true of speakers from very large corporations who follow a typical corporate structure where profit is the name of the game. The parts quality and cabinetry can definitely matter, and many benefit from bracing and better parts.