UPVC window formers Vs on site made timber profiles

[flanagaj]

When I did a house refurb / extension many years ago, I knocked up window / door opening profiles using 4x2, and the timber reused for first fix stud work when the shell was water tight.  I see that you can now get pre made UPVC window formers that are essentially a former / cavity closer in one.  On paper these look useful, but just wondering whether anyone has first hand experience of them and whether they are worth switching over to.  Just concerned that they might be flimsy and not remain very square and require bracing anyway.

Edited 18 hours ago by flanagaj