That in general is incorrect. Granted Briggs and Stratton make some cheap engines that are sometimes mated to cheap chassis's (sometimes plastic) for the likes of B&Q, but equally Briggs and Stratton make expensive engines that can be found on the more professional/expensive mower. I got one for my dad last year that was the best part of £500 and that as a Briggs engine on it at 190cc and its a corking engine very powerful.
Thank you for saying that, you saved me. We've always had a good run from our B & S's. One of our mowers, has done over 300 hours, that's a lot of mowing, and it's still running strong, always serviced properly, admittedly it's more your professional type mower. Had similar from our others too. Reasonably engineered, but like most things they're designed by boffins, that don't have much practical working/usage knowledge. Our ones are all made in Aussie. As with most things in life, you get what you pay for.