Not particularly?
A lot of the issues where Danny "cant" fix a speaker is where a tweeter can only play down to say 2000-3000hz and a woofer has severe breakup at or before it can meet the tweeter. It's a flaw with the drivers rather than his ability. The two Martin Logan AMT bookshelf speakers he looked at had this issue.
Sometimes he can make it "work" but it's less than an ideal or requires a ton of parts.
Other times the speaker is too small/tight to fit an upgraded crossover into them. The Harbeth P3ESR and Kef LS50(Meta) and a few others he's looked at had this issue.
We've also looked at the Kef R11 towers and there's too much bracing to fit a new crossover into it. If we designed a new network for it, it would be tough to install it internally. Stock crossovers are also glued to pegs on the side walls, so removing them is a destructive process.
External crossovers would be the only "real" option.
The last issue will have to do with driver spacing, impedance, or cabinet design issues
There are some speakers where the tweeter is crossed over at 2-3KHz but the mid/woofer its crossed to is several inches away from the tweeter, so any movement vertically, shifts the phase rather dramatically.
Edge diffraction is also a major issue on some models that is difficult to deal with without adding layers of felt around the drivers, and even then it's still not an ideal solution.
Other times, we cant get the drivers out without risking damage to the speaker or drivers.
In most cases it's just not worth the cost trying to fix the issues a speaker has, especially when it has issues with the drivers themselves, and will require a ton of parts to fix, which may not easily fit into a cheap speaker.
It really just depends on how well a speaker was designed and how well behaved the drivers are.
