TLDR answer to your question: Uninformed (ie without sufficient context) group think with little-to-no first hand knowledge of what they're talking about, IMO.
Longer answer:
If we are talking about bone stock 4L60Es, particularly those built before 2001, then yes, the factory handicapped them to a large extent when it comes to being ready for performance as-is compared to a 4L80E in stock form. However, with a few relatively minor changes, they can be built to handle power within reasonable limits (i..e 500 hp or less is a general consensus guideline amongst most of us) before you have spend a near infinite amount of money to get them to hold up.
Stock 4L60Es experience chronic, repeated failures of the 3-4 clutch due to its poor design and set up. This probably drives the majority of the voices telling everyone 4L60Es are made of glass and to use a 4L80 for anything beyond stock applications (and can't blame them if so). But context is everything and weighing cost/benefits when it comes to swaps is critical to making the right call...So here's more context:
4L60E Factory Design Flaws that have been driving the negative opinion/views on their durability:
> Under-designed 3/4 clutch; smaller surface apply area and not enough individual frictions to make up for it compared to peer transmissions from Ford and Dodge (AOD/4R70/75W and Dodge 46RH/RE) - makes 3/4 pack very vulnerable to premature failure
> Insufficiently sized feed orifices in the spacer plate for 1-2, 2-3, 3-4 shift feeds - lengthens overall dwell time between clutch command and full compression shortening clutch pack life
> Undersized 2nd gear apply servo piston (553 servo) installed in majority of applications (corvette servo began to appear in Corvettes, F-bodies and later proliferated to high-end sub-models of some of the trucks and SUVs in later years) combined with excessive band clearance and a band that's too narrow relative to drum surface area (shorter 2-4 band life then otherwise)
> Excessive clearance in 3-4 clutch pack (.060-.080) combined with unnecessary load release spring assemblies (intended to solve a non-existent problem of centrifugal apply while creating a very real one in premature clutch burn up)
> Decision to use a steel input shaft pressed into an aluminum drum with no reinforcement at shaft/drum junction, resulting in drum leaking at apply circuits in shaft
> Accumulator circuits designed to absorb too much shift energy, lengthening dwell time between clutch command and full compression (shortens clutch life)
> Weak metal used in the construction of the sun gear reaction shell resulting in numerous failures (loss of rev, 2nd and 4th gears)
> Pulse Width Modulation in TCC apply strategy too soft/gradual (95+) - shortening TC life
Simple 4L60E Corrective Measures that address all of the above and give the '60 a fighting chance in a performance application:
> Set up the 3-4 pack w/.030-.040 clearance w/load release springs or .040-.050 for more mild builds and delete the load release springs
> Truncate 1-2 and 3-4 accumulation by simply installing $40 dollar shift kits or your own mods (usually less than $10 in raw materials)
> Install a corvette (093) 2nd gear servo in place of factory 553 for all 4L60Es - $12.00
> Install a wide 2-4 band in all builds - $10-12 more than the factory spec 2-4 band (resurface existing reverse input drum to accommodate - $30)
> Drill out the 1-2, 2-3 and 3-4 feed orifices per converter stall; if factory 1600-1800 and corvette 2nd gear servo installed, go .076-.086 on the 1-2, .093-.100 on the 2-3 and .093-.100 on the 3-4
> Convert the PWM TCC apply to an on-off using a Fitzall valve designed specifically for that purpose ($9.00)
> Install a hardened/heat treated sun gear reaction shell on all overhauls - $40ish (Sonnax or TCI rollerized, reinforced shells for high HP/HD applications - $150)
> Cut lube passages in both sun gears on the planet-facing side to allow addl lube flow to planetary carrier bearings and pinion gears
> Install Sonnax fwd drum reinforcement sleeve kit on applications 450-500 HP and RPMs expected to exceed 5,000 frequently (unless Sonnax smart tech drum is used - it comes w/reinforcement installed, bolt-on 3-4 backing plate and hardened steel input shaft)
The above mods take a trans capable of dealing with 250-275 HP and allow it to comfortably handle up to 400-500HP, depending on which above mods are done and overall vehicle profile/application, etc (Sonnax Smart sun shells are used in 600+ HP street/strip vehicles and work great).
Myself and other builders I know personally have 700R4s and 4L60Es behind everything from 383 SBCs to built LS1s, 5.3s, 6Ls and the like making upwards of 500HP street/strip/race/etc and I personally have yet to actually have one come back other than occasionally for a refresh (i.e. the trans was working/functioning so the refreshes are proactive instead of reactive).
In my opinion, the cost benefit favors a 4L80E swap in any one (or more) of the following scenarios:
> Engine power / torque exceeds 550HP/TQ and / or engine torque curve peak is in the 3000-4500 neighborhood if power/torque are 450+ HP/TQ
> Vehicle will be towing/hauling in excess of 6,500-7,000 lbs+ frequently, especially out West
> Vehicle weight increases substantially over stock GVW for some reason
> Vehicle will be used predominantly in a HD specialized application such as snow plow trucks
> Engine will spin more than 7,000 RPMs consistently (full race, mud bogging, etc)
4L80E Benefits: With minimal mods the 4L80E can handle 750-800HP/Tq...I typically just dual feed the direct clutch, install a billet rollerized forward hub (Costliest mod at only $150-$200), rollerized output shaft-case thrust, shim the two planets as well as the forward hub-direct drum thrust to take out the slop, block off the 2-3 and 3-4 accumulator plate, install a Chrysler A518 direct clutch snap ring in place of the stock int. snap ring, install high rate return springs in the intermediate and direct clutch piston, drill out various lube orifices (free), install a Sonnax boost valve, CVBs AFL and TCC reg valve, drill the spacer plate per vehicle application, weight, final drive, stall, desired shift firmness and set clutch clearances as they need to be. Beyond 800 or so, you'll be installing lots of billet parts and bill goes up by literally $2500-3000 or more when power levels approach that 800 HP mark compared to what it takes to prep one for sub-800HP vehicles.
4L80E Swap cost drivers: You have to integrate the 4L80e into a vehicle that originally took a 4L60 (Driveline cut/rebalanced, x member moved, etc), reprogram the PCM via segment swap (HP Tuners) and purchase an external harness adaptor. 4L80E rebuild kit and parts are appx 40% higher than a 4L60e (bushings, clutch and steel modules, bands, paper, rubber, seals, one-way clutch assemblies, etc)
Additionally, the 4L80E consumes about 15% more crank horse power to turn than a 4L60 so that by itself makes me hesitate do encourage swaps outside of specialized situations where engine power is 450HP or less, which is the majority of engines that would be mated to either transmission. A 4L60E can be built for much less overall combined costs and will work just fine if done correctly and vehicle/trans is well maintained.
That should bring a more balanced perspective to the issue while clearing up some of the confusion around what folks typically say about the 60 for anything beyond stock and why some builders like myself push back a little on that notion.
As always, context is everything.
Hope it helps...