I know a lot about suits.
We need to first define what a "designer" suit is.
People who wear nice off-the-rack suits don't call them "designer" suits. I'm talking Brioni, Kiton, Isaia and so on. If this is what you mean then ok. The "designer" suits are mostly slim cut short tail crap that just looks stupid, even though people wearing them may think they look trendy and that they are "customized" for you. I'm talking crap like Hugo Boss, Armani, Zegna, Ralph Lauren etc. There are a few exceptions from these brands (like RLPL). Most of these are made from average fabrics (Reda, V. Barberis Canonico and such. Ok'ish, but far from the best), and their construction is cheap (fused or half-canvassed v. full horsehair canvas construction, machine sown seams, sleeves not applied by hand, things like that).
If you are going to spend a decent amount of money on a nice suit, avoid all the "designer" stuff and just go bespoke (bespoke, not the made-in-asia MTM stuff. MTM is a good option if you know what you want and how to make it happen). You'll get something that fits you well, will last a lifetime, and looks good, always.